Rework mod enumeration / caching.

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

OpenRA.Game/IUtilityCommand.cs

@@ -16,10 +16,12 @@ namespace OpenRA
 	public class Utility
 	{
 		public readonly ModData ModData;
+		public readonly InstalledMods Mods;
 
-		public Utility(ModData modData)
+		public Utility(ModData modData, InstalledMods mods)
 		{
 			ModData = modData;
+			Mods = mods;
 		}
 	}