Implement mod-defined package loaders.

OpenRA.Game/FileSystem/IPackage.cs

@@ -15,12 +15,23 @@ using System.IO;
 
 namespace OpenRA.FileSystem
 {
+	public interface IPackageLoader
+	{
+		/// <summary>
+		/// Attempt to parse a stream as this type of package.
+		/// If successful, the loader is expected to take ownership of `s` and dispose it once done.
+		/// If unsuccessful, the loader is expected to return the stream position to where it started.
+		/// </summary>
+		bool TryParsePackage(Stream s, string filename, FileSystem context, out IReadOnlyPackage package);
+	}
+
 	public interface IReadOnlyPackage : IDisposable
 	{
 		string Name { get; }
 		IEnumerable<string> Contents { get; }
 		Stream GetStream(string filename);
 		bool Contains(string filename);
+		IReadOnlyPackage OpenPackage(string filename, FileSystem context);
 	}
 
 	public interface IReadWritePackage : IReadOnlyPackage