WIP: _casbaseddirectory.py: Do not resolve symlinks

This matches the change in utils._process_list().

This also removes the _Resolver class as it is now unused. We may want
to support controlled symlink resolution in the future, in which case
the _Resolver class can be resurrected from this commit.

WIP: CAS import tests are marked as xfail as they haven't been updated
yet to match the new behavior.