[generator] [Register] interface alternatives into a dummy package (#1242)

Fixes: #802

Context: #798 (comment)

Context: a3b7456

C# and Java are (famously) different languages, and have (had?)
different opinions about e.g. what could be within an `interface`
declaration.  Consider `java.lang.reflect.Member`:

	// Java
	public interface Member {
	    public static final int PUBLIC      = 0;
	    public static final int DECLARED    = 1;

	    public Class<?> getDeclaringClass();
	    public String   getName();
	    public int      getModifiers();
	    public boolean  isSynthetic();
	}

How do we bind this in C#?

Before C# 8 introduced default interface members, we would bind it
as *two* types: the interface binding, and an "interface alternative":

	// C# 7
	[Register ("java/lang/reflect/Member", "", "Java.Lang.Reflect.IMemberInvoker")]
	public interface IMember : IJavaPeerable {
	    Java.Lang.Class DeclaringClass  {get;}
	    bool            IsSynthetic     {get;}
	    int             Modifiers       {get;}
	    string          Name            {get;}
	}

	// interface alternative
	[Register ("java/lang/reflect/Member", DoNotGenerateAcw=true)]
	public abstract class Member : Java.Lang.Object {
	    internal Member(); /* prevent subclassing */

	    public const int Declared   = 1;
	    public const int Public     = 0;
	}

An "interface alternative" is a class with the same name as the Java
class, containing all the interface members which couldn't be placed
within the interface declaration: static fields, static methods, ….

C# 8 allowed `interface`s to contain non-instance members, and with
commit a3b7456 this could instead be bound as:

	// C# 8
	[Register ("java/lang/reflect/Member", "", "Java.Lang.Reflect.IMemberInvoker")]
	public partial interface IMember : Java.Interop.IJavaPeerable {
	    public const int Declared = 1;
	    public const int Public = 0;

	    Java.Lang.Class DeclaringClass  {get;}
	    bool            IsSynthetic     {get;}
	    int             Modifiers       {get;}
	    string          Name            {get;}
	}
	// No interface alternative

However, while this was *possible*, removing the
"interface alternative" would be an API break, and thus was retained
for existing types (bd7c60a).

There is one issue with the interface alternative, as previously
declared: the `[Register]` attribute was present, and declared the
same Java name as the interface binding:

	[Register ("java/lang/reflect/Member", "", "Java.Lang.Reflect.IMemberInvoker")]
	public partial interface IMember {}

	[Register ("java/lang/reflect/Member", DoNotGenerateAcw=true)]
	public partial class Member {}

which in turn means that often there isn't a 1:1 mapping between
Java types and C# types, but instead a 1:2 mapping, an ambiguity.

This in turn can creep into [binding projects][0] and cause the
generation of invalid code:

	public partial class JSONArray
	    : global::Com.Alibaba.Fastjson.JSON
	    , global::Java.IO.ISerializable
	    , global::Java.Lang.ICloneable
	    , global::Java.Util.List
	    , global::Java.Util.IRandomAccess
	{}

`Java.Util.List` is the interface alternative for the `java.util.List`
interface type, which is a class.  The above thus fails to compile
with a CS1721 ("Class 'JSONArray' cannot have multiple base classes:
'JSON' and 'List'").  `JSONArray` should instead have implemented
the `Java.Util.IList` *interface*, but didn't because of this
ambiguity.

Remove this ambiguity by instead registering a "fake" class for
interface alternatives, using a `mono/internal/` prefix:

	// interface declaration is unchanged
	[Register ("java/lang/reflect/Member", "", "Java.Lang.Reflect.IMemberInvoker")]
	public partial interface IMember {}

	// interface alternative registers a non-existent type
	[Register ("mono/internal/java/lang/reflect/Member", DoNotGenerateAcw=true)]
	public partial class Member {}

*Note*: `[Register]` must still be used, because if we remove
`[Register]` entirely, then `jcw-gen` will attempt to create a new
Java Callable Wrapper for it, as interface alternatives are still
`Java.Lang.Object` subclasses.

[0]: dotnet/android#9188 (comment)

Author: jpobst

tests/generator-Tests/Unit-Tests/CodeGeneratorExpectedResults/XAJavaInterop1-NRT/WriteInterface.txt

@@ -1,4 +1,4 @@
-[Register ("java/code/IMyInterface", DoNotGenerateAcw=true)]
+[Register ("mono/internal/java/code/IMyInterface", DoNotGenerateAcw=true)]
 public abstract class MyInterface : Java.Lang.Object {
 	internal MyInterface ()
 	{
@@ -19,7 +19,7 @@ public abstract class MyInterface : Java.Lang.Object {
 
 }
 
-[Register ("java/code/IMyInterface", DoNotGenerateAcw=true)]
+[Register ("mono/internal/java/code/IMyInterface", DoNotGenerateAcw=true)]
 [global::System.Obsolete (@"Use the 'MyInterface' type. This type will be removed in a future release.", error: true)]
 public abstract class MyInterfaceConsts : MyInterface {
 	private MyInterfaceConsts ()

tests/generator-Tests/Unit-Tests/CodeGeneratorExpectedResults/XAJavaInterop1/ObsoleteInterfaceAlternativeClass.txt

@@ -1,4 +1,4 @@
-[Register ("com/xamarin/android/Parent", DoNotGenerateAcw=true)]
+[Register ("mono/internal/com/xamarin/android/Parent", DoNotGenerateAcw=true)]
 [global::System.Obsolete (@"Use the 'Com.Xamarin.Android.IParent' type. This class will be removed in a future release.")]
 public abstract class Parent : Java.Lang.Object {
 	internal Parent ()

tests/generator-Tests/Unit-Tests/CodeGeneratorExpectedResults/XAJavaInterop1/WriteConstSugarInterfaceFields.txt

@@ -1,4 +1,4 @@
-[Register ("java/code/IMyInterface", DoNotGenerateAcw=true)]
+[Register ("mono/internal/java/code/IMyInterface", DoNotGenerateAcw=true)]
 public abstract class MyInterface : Java.Lang.Object {
 	internal MyInterface ()
 	{

tests/generator-Tests/Unit-Tests/CodeGeneratorExpectedResults/XAJavaInterop1/WriteInterface.txt

@@ -1,4 +1,4 @@
-[Register ("java/code/IMyInterface", DoNotGenerateAcw=true)]
+[Register ("mono/internal/java/code/IMyInterface", DoNotGenerateAcw=true)]
 public abstract class MyInterface : Java.Lang.Object {
 	internal MyInterface ()
 	{
@@ -19,7 +19,7 @@ public abstract class MyInterface : Java.Lang.Object {
 
 }
 
-[Register ("java/code/IMyInterface", DoNotGenerateAcw=true)]
+[Register ("mono/internal/java/code/IMyInterface", DoNotGenerateAcw=true)]
 [global::System.Obsolete (@"Use the 'MyInterface' type. This type will be removed in a future release.", error: true)]
 public abstract class MyInterfaceConsts : MyInterface {
 	private MyInterfaceConsts ()

tests/generator-Tests/Unit-Tests/CodeGeneratorExpectedResults/XAJavaInterop1/WriteInterfaceFieldAsDimProperty.txt

@@ -1,4 +1,4 @@
-[Register ("com/xamarin/android/MyInterface", DoNotGenerateAcw=true)]
+[Register ("mono/internal/com/xamarin/android/MyInterface", DoNotGenerateAcw=true)]
 public abstract class MyInterface : Java.Lang.Object {
 	internal MyInterface ()
 	{
@@ -32,7 +32,7 @@ public abstract class MyInterface : Java.Lang.Object {
 
 }
 
-[Register ("com/xamarin/android/MyInterface", DoNotGenerateAcw=true)]
+[Register ("mono/internal/com/xamarin/android/MyInterface", DoNotGenerateAcw=true)]
 [global::System.Obsolete (@"Use the 'MyInterface' type. This type will be removed in a future release.", error: true)]
 public abstract class MyInterfaceConsts : MyInterface {
 	private MyInterfaceConsts ()

tests/generator-Tests/Unit-Tests/CodeGeneratorExpectedResults/XAJavaInterop1/WriteStaticInterfaceMethod.txt

@@ -1,4 +1,4 @@
-[Register ("java/code/IMyInterface", DoNotGenerateAcw=true)]
+[Register ("mono/internal/java/code/IMyInterface", DoNotGenerateAcw=true)]
 public abstract class MyInterface : Java.Lang.Object {
 	internal MyInterface ()
 	{
@@ -19,7 +19,7 @@ public abstract class MyInterface : Java.Lang.Object {
 
 }
 
-[Register ("java/code/IMyInterface", DoNotGenerateAcw=true)]
+[Register ("mono/internal/java/code/IMyInterface", DoNotGenerateAcw=true)]
 [global::System.Obsolete (@"Use the 'MyInterface' type. This type will be removed in a future release.", error: true)]
 public abstract class MyInterfaceConsts : MyInterface {
 	private MyInterfaceConsts ()

tests/generator-Tests/expected.xaji/TestInterface/Test.ME.ITestInterface.cs

@@ -5,7 +5,7 @@
 
 namespace Test.ME {
 
-	[Register ("test/me/TestInterface", DoNotGenerateAcw=true)]
+	[Register ("mono/internal/test/me/TestInterface", DoNotGenerateAcw=true)]
 	public abstract class TestInterface : Java.Lang.Object {
 		internal TestInterface ()
 		{
@@ -31,7 +31,7 @@ internal TestInterface ()
 
 	}
 
-	[Register ("test/me/TestInterface", DoNotGenerateAcw=true)]
+	[Register ("mono/internal/test/me/TestInterface", DoNotGenerateAcw=true)]
 	[global::System.Obsolete (@"Use the 'TestInterface' type. This type will be removed in a future release.", error: true)]
 	public abstract class TestInterfaceConsts : TestInterface {
 		private TestInterfaceConsts ()

tools/generator/SourceWriters/InterfaceMemberAlternativeClass.cs

@@ -39,7 +39,8 @@ public InterfaceMemberAlternativeClass (InterfaceGen iface, CodeGenerationOption
 
 			SourceWriterExtensions.AddSupportedOSPlatform (Attributes, iface, opt);
 
-			Attributes.Add (new RegisterAttr (iface.RawJniName, noAcw: true, additionalProperties: iface.AdditionalAttributeString ()) { AcwLast = true });
+			// Place this class in a dummy package that won't conflict the actual interface
+			Attributes.Add (new RegisterAttr ("mono/internal/" + iface.RawJniName, noAcw: true, additionalProperties: iface.AdditionalAttributeString ()) { AcwLast = true });
 
 			if (should_obsolete)
 				SourceWriterExtensions.AddObsolete (Attributes, $"Use the '{iface.FullName}' type. This class will be removed in a future release.", opt);
@@ -179,7 +180,9 @@ public InterfaceConstsForwardClass (InterfaceGen iface, CodeGenerationOptions op
 			IsPublic = true;
 			IsAbstract = true;
 
-			Attributes.Add (new RegisterAttr (iface.RawJniName, noAcw: true, additionalProperties: iface.AdditionalAttributeString ()));
+			// Place this class in a dummy package that won't conflict the actual interface
+			Attributes.Add (new RegisterAttr ("mono/internal/" + iface.RawJniName, noAcw: true, additionalProperties: iface.AdditionalAttributeString ()));
+
 			SourceWriterExtensions.AddObsolete (Attributes, $"Use the '{iface.Name.Substring (1)}' type. This type will be removed in a future release.", opt, isError: true);
 
 			Constructors.Add (new ConstructorWriter {