Support: Remove duplicated code in {File,clang::ModulesDependency}Collector, NFC

Refactor the duplicated canonicalize-path logic in `FileCollector` and
`ModulesDependencyCollector` into a new utility called
`PathCanonicalizer` that's shared. This popped up when tracking down a
bug common to both in https://reviews.llvm.org/D95202.

As drive-bys, update a few names and comments to better reflect the
effect of the code, delay removal of `..`s to avoid an unnecessary extra
string copy, and leave behind a couple of FIXMEs for future
consideration.

Differential Revision: https://reviews.llvm.org/D95279