Support total conversion mods

[mugling]

Groundwork for supporting total conversion mods. A conceptually much simpler approach than the previous whitelisting attempts (which were complex and contained many bugs).

• Initial cleanup of mod loading code
• Adds support for core content packs which are loaded before all other content
• dda is the first core pack and also enabled by default for all games
• Unit tests and --dump-stats both use the dda core content pack
• Almost all mods currently depend upon dda
• Fully backward compatible with all savegames

Next step is to move the minimal number of items from dda to data/core and then other contributors can begin to write total conversion mods.

[Rivet-the-Zombie]

This seems like the inevitable evolution of our mod manager. Cool idea!

[mugling]

Added bonus: non-content mods (for example fast zombies) are now much quicker via --check-mods

[Coolthulhu]

--check-mods spews many pages of errors.

[mugling]

Found it - some mods were lacking a dependency on dda - all mods are going to require this until we have moved the minimal definitions required to prevent load errors to core. Should be fixed by latest commit.