This abstracts away the canonicalizer and solves the problem whereby
canonicalization was not being performed prior to recording whether a path
has been visited. This ensures that multiple relative paths to the same
file will be properly recognized as visited.
This will be entirely replaced in an upcoming commit. See that for
details. I don't feel like dealing with the conflicts for rearranging and
squashing these commits.
This also includes an implementation to visit paths only once. Note that it
does not yet canonicalize the path before visiting, so relative paths to the
same file can slip through, and relative paths to _different_ files could be
erroneously considered to have been visited.
This will be fixed in an upcoming commit.
Mike Gerwitz