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/ImportLegacyMapCommand.cs

@@ -74,7 +74,7 @@ namespace OpenRA.Mods.Common.UtilityCommands
 					Author = "Westwood Studios",
 				};
 
-				Map.RequiresMod = ModData.Manifest.Mod.Id;
+				Map.RequiresMod = ModData.Manifest.Id;
 
 				SetBounds(Map, mapSection);