Support .mod file dependencies in loader

[rquinio]

Based on these mods:

• BaseMod.mod, dependencies = {}
• SubMod1.mod, dependencies = { BaseMod }
• SubMod2.mod, dependencies = { BaseMod SubMod1 }

If there is a file with identical name (.gfx, .dds, etc.), the priority should likely be:
SubMod2 > SubMod1 > BaseMod > Vanilla+DLCs
To be confirmed with some testing.

Without dependencies, the priority would be first mod loaded wins (BaseMod > SubMod1 > SubMod2 > > Vanilla+DLCs)

Not sure how games behaves when there's a dependency cycle.