[generator] better support for package-private interfaces

[gugavaro]

Java allows public classes/interfaces to inherit/implements from "package-private" interfaces, in
which case the public & protected members of the "package-private"
interface are visible within the public class:

    // Java
    /* package */ interface BaseInterface {
     void foo();
      // ...
    }
    public interface TestInterface extends BaseInterface {
      // ...
   }
   public class TestClass implements TestInterface {
      // ...
    }

or

    public class TestClass implements BaseInterface {
      // ...
    }
 

The problem is that generator didn't properly support this construct,
and skips binding of "package-private" types, resulting in generated
C# code such as:

// C#
public partial interface TestInterface : BaseInterface {
  // CS0234: The type or namespace name `BaseInterface` does not exist in the namespace
}

Support this construct by updating generator to "copy" the members
from the "package-private" interface into the declaring interface:

// C#
public partial class TestClass : Java.Lang.Object {
  public void Foo() {
  // ...
}

This allows the generated code to compile without metadata fixup.

Specifics in implementing this:

• Add a InterfaceGen.FixupAccessModifiers() to fix the private interfaces
• Call FixupAccessModifiers() in the "validation" step
• Add a property to expose the ImplementedInterfaces
• In FixupAccessModifiers(), lookup the base interface of the current
  type and check if it is non-public.
• Skip the package-private types, and replace them with that
  interface's base type
• Look for each method on the base type, and copy it to the public
  type if it does not exist
• Added tests for the follow scenarios
  • Private Interface -> Public Interface -> Public Class
  • Private Interface -> Public Class