Refactor: Replace custom path utilities with Path API and update zip methods accordingly

[moritz-gross]

• reduces LOC by 10
• having proper types instead of str is safer imo

Side note: This file also contains some unused imports. should I remove them as well?

and I'm surprised somewhat that the previous implementation is not buggy in some way?? hard-coding a path separator like "/" is usually a bad idea from my experience, given the kinds of this operating systems are doing.

[NSoiffer]

@moritz-gross Except for some special cases, "/" works everywhere in Rust. There is MAIN_SEPARATOR, but is obviously bulkier and less readable.

Please test this with feature = "include-zip". There is code in shim-filesystem.rs that wan't to have "/"s due to dealing with zip files.

[moritz-gross]

cargo test --features include-zip passes

(I'm getting this warning though, which is fine I guess: warning: methods rule_files_cache_is_empty, definitions_files_cache_is_empty, and definitions_files_cache_path are never used )

I've not used the flag --features include-zip before.