Allow the ra/cnc package dir to be overridden at launch-time.

OpenRA.FileFormats/Filesystem/FileSystem.cs

@@ -19,6 +19,7 @@ namespace OpenRA.FileFormats
 	public static class FileSystem
 	{
 		static List<IFolder> mountedFolders = new List<IFolder>();
+		public static string SpecialPackageRoot = "";
 
 		static Cache<uint, List<IFolder>> allFiles = new Cache<uint, List<IFolder>>( _ => new List<IFolder>() );
 
@@ -73,7 +74,11 @@ namespace OpenRA.FileFormats
 		{
 			var optional = name.StartsWith("~");
 			if (optional) name = name.Substring(1);
-
+			
+			// paths starting with $ are relative to SpecialPackageRoot
+			if (name.StartsWith("$"))
+				name = SpecialPackageRoot+name.Substring(1);
+			
 			var a = (Action)(() => FileSystem.MountInner(OpenPackage(name)));
 
 			if (optional)

OpenRA.Game/Game.cs

@@ -219,6 +219,8 @@ namespace OpenRA
 												+ Path.DirectorySeparatorChar + "OpenRA";
 
 			SupportDir = args.GetValue("SupportDir", defaultSupport);
+			FileSystem.SpecialPackageRoot = args.GetValue("SpecialPackageRoot", "");
+			
 			Settings = new Settings(SupportDir + "settings.yaml", args);
 
 			Settings.Save();

mods/cnc/mod.yaml

@@ -9,7 +9,7 @@ Folders:
 	mods/cnc
 	mods/cnc/bits
 	mods/cnc/uibits
-	mods/cnc/packages
+	$mods/cnc/packages
 
 Packages:
 	overrides.mix

mods/ra/mod.yaml

@@ -10,7 +10,7 @@ Folders:
 	mods/ra
 	mods/ra/bits
 	mods/ra/uibits
-	mods/ra/packages
+	$mods/ra/packages
 
 Packages:
 	~main.mix