[clang][deps] Avoid leaking modulemap paths across unrelated imports

Use a FileEntryRef when retrieving modulemap paths in the scanner so
that we use a path compatible with the original module import, rather
than a FileEntry which can allow unrelated modules to leak paths into
how we build a module due to FileManager mutating the path.

Note: the current change prevents an "unrelated" path, but does not
change how VFS mapped paths are handled (which would be calling
getNameAsRequested) nor canonicalize the path.

Differential Revision: https://reviews.llvm.org/D137989