Rework `beets.util.hidden`

[bal-e]

The implementation was unnecessarily convoluted. I've rewritten it to declare fewer functions, and it now uses pathlib.

P.S. I'm starting to feel bad for opening so many PRs :P

[github-actions[bot]]

Thank you for the PR! The changelog has not been updated, so here is a friendly reminder to check if you need to add an entry.

[Serene-Arc]

Don't feel bad! It's great to see someone interested in the repo and contributing :)

[bal-e]

bump -- let me know if there's anything else left to do.

[bal-e]

Thanks @Serene-Arc!