Handle loaded MPQ files in a map instead of bespoke globals

[AJenbo]

This is done as prep work for being able to load and unload mods that are distributed as MPQ files.

The changes here are done in a way that changes as little as possible about how things works.

• bespoke bools are still used to track what was loaded
• Hellfire is always loaded but it's files are skipped if not active
• priority values match the original Diablo/Hellfire

In a follow up to this PR I would like to:

• mods are automatically loaded in to the 1xxxx range (the specific value will depend on what order they appear in the ini).
• 1xxx and 9xxxx stays loaded but changing the mod will reload the 8xxx and 1xxxx range.
• Hellfire is loaded via a lua function

[StephenCWills]

Small note for glebm: Looks like with this change we are checking ASSETS_VERSION on systems with UNPACKED_MPQS where we were not before. So long as the ASSETS_VERSION file gets deployed on those platforms, I don't see this as an issue.

[AJenbo]

Thanks :)