Allow using a list of paths in emacsWithPackagesFromUsePackage

[TLATER]

This permits creating use-package/leaf package lists for full configuration directories, instead of just single big config files, at least unless I misunderstand something quite a bit.

[TLATER]

This can be done downstream, of course, but I think it's neater to have actual support for it.

[TLATER]

This... Doesn't seem to work in practice, somehow, I'd only tested it without going through the full entrypoint. Somehow going here makes nix not resolve the <thunk>s pointing into my source into strings before passing them to builtins.readFile, which breaks everything.

Unsure how this is even possible, but I'll have to dig into this a bit deeper than I can right this evening.