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/OpenRA.Game.csproj

@@ -245,6 +245,7 @@
     <Compile Include="FileSystem\ZipFolder.cs" />
     <Compile Include="Primitives\float3.cs" />
     <Compile Include="ModContent.cs" />
+    <Compile Include="InstalledMods.cs" />
   </ItemGroup>
   <ItemGroup>
     <Compile Include="FileSystem\D2kSoundResources.cs" />