Optimize enemy loading

[WilliamFr0g]

Probably still kinda bad, but it's a lot less bad

I only just quickly swapped some calls for this to work in love 11 because it was built in love 10, it has not been formally tested

[alesan99]

Haven't tested yet but some case-sensitivity code should be fixed and re-added

[qixils]

Given how many times enemies are (re)loaded I definitely think we should have a perpetual in-memory cache (along the lines of a table mapping filepaths to enemies) somewhere to really optimize enemy loading, or at least smarter logic for when to clear the customenemies table, as currently enemies are still loaded on mappack change as well as reloaded on level change. So while this PR certainly cleans up the code and resolves issues, it doesn't feel especially faster (although I haven't benchmarked it, I see the code for it and trust that it was done). However I think it would make sense to save that for a future PR, as managing the cache alongside the base system seems nontrivial.

Tested briefly and everything seems functional to me.

EDIT: Did do a quick benchmark; I get about 0.3s (x2=0.6s) to load the Kirby mappack's enemies on this branch, and 0.4s (x2=0.8s) on the master branch. 33% improvement isn't too bad, but still a total of 0.6s being spent to actually get into a level in the mappack, with half of that being a useless reload, so definitely room to improve in the future.