Rework mod enumeration / caching.

- Replaced ModMetadata.AllMods with Game.Mods.
- Store / reference mod Manifest instead of ModMetadata.
- Removes engine dependency on ModContent class.

OpenRA.Mods.Common/UtilityCommands/ListMixContentsCommand.cs

@@ -31,7 +31,7 @@ namespace OpenRA.Mods.Common.UtilityCommands
 			var filename = Path.GetFileName(args[1]);
 			var path = Path.GetDirectoryName(args[1]);
 
-			var fs = new OpenRA.FileSystem.FileSystem();
+			var fs = new FileSystem.FileSystem(utility.Mods);
 
 			// Needed to access the global mix database
 			fs.LoadFromManifest(utility.ModData.Manifest);