[jcwgen] add public method for GetDestinationPath (#402)

Context: dotnet/android#2505 (comment)

Xamarin.Android is working to reduce the number of temporary files
which are generated during the build, in order to improve build time.
We would like `JavaCallableWrapperGenerator` to assist in that effort.

Currently we are doing:

	// jti is a JavaCallableWrapperGenerator instance
	jti.Generate (outputPath); 

	// Then later in our MSBuild task
	foreach (var file in Directory.GetFiles (temp, "*", SearchOption.AllDirectories)) {
	    ...
	    MonoAndroidHelper.CopyIfChanged (file, dest);
	}

This is slower than it could be:

 1. Writes to a bunch of temp files.
 2. Recurses the directory where all these temp files exist.
 3. Does a hash comparison of the contents of the source and
    destination.
 4. Copies the file if needed.
 5. Deletes all the temp files.

Instead, we want to instead do:

	using (var memoryStream = new MemoryStream ())
	using (var writer       = new StreamWriter (memoryStream)) {
	    jti.Generate (writer);
	    writer.Flush ();
	    var path = jti.GetDestinationPath (outputPath);
	    MonoAndroidHelper.CopyIfStreamChanged (memoryStream, path);
	}

This is more streamlined:

 1. Generate the file in-memory.
 2. Do a hash comparison of the in-memory file vs. the destination.
 3. Write to the file if needed.

The change we need in Java.Interop here is to add the
`JavaCallableWrapperGenerator.GetDestinationPath()` method.  Further
changes downstream in xamarin-android will allow us reap the benefits.

I also improved the code a bit, no longer using `string.Split()`.
We can do a simple `string.Replace()` from `.` to
`Path.DirectorySeparatorChar`, which allocates fewer strings.

Author: jonathanpeppers

src/Java.Interop.Tools.JavaCallableWrappers/Java.Interop.Tools.JavaCallableWrappers/JavaCallableWrapperGenerator.cs

@@ -856,15 +856,18 @@ void WriteInstrumentationOnCreate (TextWriter sw, Action<TextWriter> extra)
 
 		StreamWriter OpenStream (string outputPath)
 		{
-			string path = outputPath;
-			foreach (string dir in package.Split ('.'))
-				path = Path.Combine (path, dir);
-
-			if (!Directory.Exists (path))
-				Directory.CreateDirectory (path);
+			string destination = GetDestinationPath (outputPath);
+			Directory.CreateDirectory (Path.GetDirectoryName (destination));
+			return new StreamWriter (new FileStream (destination, FileMode.Create, FileAccess.Write));
+		}
 
-			path = Path.Combine (path, name + ".java");
-			return new StreamWriter (new FileStream (path, FileMode.Create, FileAccess.Write));
+		/// <summary>
+		/// Returns a destination file path based on the package name of this Java type
+		/// </summary>
+		public string GetDestinationPath (string outputPath)
+		{
+			var dir = package.Replace ('.', Path.DirectorySeparatorChar);
+			return Path.Combine (outputPath, dir, name + ".java");
 		}
 	}
 }