Mod and mod-mounting cleanup and improvements.

• Non-visual replays now automatically try to load the replay mods. This removes the annoyance that -mod=public usually had to be passed.
• MountMods is no longer called in InitVfs but later, making it possible to load the game in one pass & simplifying things considerably.
• Explicitly ignore duplicates when loading mods
• Interface cleanup: failed mods and incompatible mods were redundant, only incompatible mods is kept.
• Interface cleanup: AreModsCompatiblebecomes CheckForIncompatibleMods, which becomes a private pure function, simplifying the control flow somewhat.
• Interface cleanup: CheckAndEnableMods is just EnableMods, which explicitly updates loaded & incompatible mods.
• CacheEnabledModVersions becomes private and is called on behalf of the user, removing the need to be careful about updating that (fixes my concern at rP21823)

Overall, the logic around mounting & enabled mods should be easier to understand.

Differential Revision: https://code.wildfiregames.com/D3982