When dep-scanning, canonicalize the module map path as much as we can.
This avoids unnecessarily needing to build multiple versions of a module
due to symlinks or case-insensitive file paths.

Despite the name `tryGetRealPathName`, the previous implementation did
not actually return the realpath most of the time, and indeed it would
be incorrect to do so since the realpath could be outside the module
directory, which would have broken finding headers relative to the
module.

Instead, use a canonicalization that is specific to the needs of
modulemap files (canonicalize the directory separately from the
filename).

Differential Revision: https://reviews.llvm.org/D134923