Replace Path with std::filesystem::path in libfetchers.

[vinayakankugoyal]

Motivation

Replace Path with std::filesystem::path in libfetchers.

Context

#9205

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[Ericson2314]

       > ../globals.cc:489:9: error: no matching function for call to 'hasPrefix'
       >   489 |     if (hasPrefix(defaultTempDir(), "/var/folders/"))
       >       |         ^~~~~~~~~

Can do a bit nicer with https://en.cppreference.com/w/cpp/filesystem/path/lexically_normal

[Ericson2314]

@xokdvium let me know if you think my makeFSSourceAccessor suggestions are too much for now because potential symlink issues.