[xabuild] MSBuild assemblies should not "Copy Local"

Mono 2018-04 (or 5.14.x) has some changes to MSBuild assemblies that
cause the following failure when using `xabuild.exe`:

    Mono: The following assembly referenced from monodroid/external/xamarin-android/bin/Debug/bin/Microsoft.Build.Tasks.Core.dll could not be loaded:
        Assembly:   System.Reflection.Metadata    (assemblyref_index=7)
        Version:    1.3.0.0
        Public Key: b03f5f7f11d50a3a

Currently `xabuild.exe` is referencing various MSBuild assemblies that
get copied into `bin/$(Configuration)/bin` and shipped in the OSS zip.

This isn't exactly desirable, since we don't really want to ship these
assemblies... These assemblies are OS-specific (as far as I can tell),
I noticed that these assemblies coming from a zip built on macOS do
not work on Windows.

So instead, we can use `Private=False` item metadata on all MSBuild
references and use custom assembly loading. By using the
`AppDomain.AssemblyResolve` event, we can add some code to probe the
MSBuild bin directory for assemblies. If not found, we can return
`null` and let the runtime do its default behavior.

Author: jonathanpeppers

build-tools/scripts/MSBuildReferences.projitems

@@ -7,18 +7,23 @@
   <ItemGroup>
     <Reference Include="Microsoft.Build">
       <HintPath>$(MSBuildReferencePath)\Microsoft.Build.dll</HintPath>
+      <Private>False</Private>
     </Reference>
     <Reference Include="Microsoft.Build.Engine">
       <HintPath>$(MSBuildReferencePath)\Microsoft.Build.Engine.dll</HintPath>
+      <Private>False</Private>
     </Reference>
     <Reference Include="Microsoft.Build.Framework">
       <HintPath>$(MSBuildReferencePath)\Microsoft.Build.Framework.dll</HintPath>
+      <Private>False</Private>
     </Reference>
     <Reference Include="Microsoft.Build.Tasks.Core">
       <HintPath>$(MSBuildReferencePath)\Microsoft.Build.Tasks.Core.dll</HintPath>
+      <Private>False</Private>
     </Reference>
     <Reference Include="Microsoft.Build.Utilities.Core">
       <HintPath>$(MSBuildReferencePath)\Microsoft.Build.Utilities.Core.dll</HintPath>
+      <Private>False</Private>
     </Reference>
   </ItemGroup>
 </Project>

tools/xabuild/XABuild.cs

@@ -1,18 +1,40 @@
-using Microsoft.Build.CommandLine;
-using System;
+using System;
 using System.IO;
 using System.Linq;
+using System.Reflection;
 using System.Threading;
 using System.Xml;
 
 namespace Xamarin.Android.Build
 {
 	class XABuild
 	{
+		static readonly XABuildPaths paths;
+
+		static XABuild ()
+		{
+			paths = new XABuildPaths ();
+
+			//NOTE: we have to do this in a static ctor, because assemblies are getting loaded before Main()
+			AppDomain.CurrentDomain.AssemblyResolve += (sender, e) => {
+				var name = new AssemblyName (e.Name);
+				var path = Path.Combine (paths.MSBuildBin, name.Name + ".dll");
+				if (File.Exists (path)) {
+					Console.WriteLine ($"[xabuild] custom assembly resolution '{e.Name}' -> '{path}'");
+					return Assembly.LoadFrom (path);
+				}
+				path = Path.Combine (paths.MSBuildBin, name.Name + ".exe");
+				if (File.Exists (path)) {
+					Console.WriteLine ($"[xabuild] custom assembly resolution '{e.Name}' -> '{path}'");
+					return Assembly.LoadFrom (path);
+				}
+				return null; //Let the default runtime behavior occur
+			};
+		}
+
 		[MTAThread]
 		static int Main ()
 		{
-			var paths = new XABuildPaths ();
 			try {
 				if (!Directory.Exists (paths.XamarinAndroidBuildOutput)) {
 					Console.WriteLine ($"Unable to find Xamarin.Android build output at {paths.XamarinAndroidBuildOutput}");
@@ -45,7 +67,7 @@ static int Main ()
 					}
 				}
 
-				int exitCode = MSBuildApp.Main ();
+				int exitCode = MSBuild.Run ();
 				if (exitCode != 0) {
 					Console.WriteLine ($"MSBuildApp.Main exited with {exitCode}, xabuild configuration is:");
 
@@ -165,4 +187,14 @@ static void SetProperty (XmlNode toolsets, string name, string value)
 			}
 		}
 	}
+
+	//HACK: While running under Mono, I had to put all MSBuild usage in a second static class.
+	//      Otherwise, I was not getting a chance to wire up AppDomain.Assembly resolve before MSBuild.dll was loaded...
+	static class MSBuild
+	{
+		public static int Run ()
+		{
+			return Microsoft.Build.CommandLine.MSBuildApp.Main ();
+		}
+	}
 }

tools/xabuild/xabuild.csproj

@@ -37,6 +37,7 @@
   <ItemGroup>
     <Reference Include="MSBuild">
       <HintPath>$(MSBuildReferencePath)\MSBuild.$(_MSBuildExtension)</HintPath>
+      <Private>False</Private>
     </Reference>
     <Reference Include="System" />
     <Reference Include="System.Core" />