Speed up GetAvailableMods for archives.
When a .zip file is encountered by the VFS population, it reads the info for all files in the archives. This is quite slow for the public archive (400-500ms on my computer), which means calling GetEngineInfo() repeatedly is impossible.
By only opening the external mod.json, we skip most of the work. The archive can still be opened if needed as fallback.
Differential Revision: https://code.wildfiregames.com/D3216