Cache: Fix findPrefix when paths are slightly out of the ordinary

This makes Cache.findPrefix/findPrefixResolved use `std.fs.path.relative` instead of `std.mem.startsWith` when checking if a file is within a prefix. This fixes multiple edge cases around prefix detection:

- If a prefix path ended with a path separator, then the first character of the 'sub_path' would get cut off because the previous implementation assumed it was a path separator. Example: prefix: `/foo/`, file_path: `/foo/abc.txt` would see that they both start with `/foo/` and then slice starting from one byte past the common prefix, ending up with `bc.txt` instead of the expected `abc.txt`
- If a prefix contained double path separators after any component, then the `startsWith` check would erroneously fail. Example: prefix: `/foo//bar`, file_path: `/foo/bar/abc.txt` would not see that abc.txt is a sub path of the prefix `/foo//bar`
- On Windows, case insensitivity was not respected at all, instead the UTF-8 bytes were compared directly

This fixes all of the things in the above list (and possibly more).

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