lib/repo-finder-mount: Change the schema for finding repos on volumes

lib/repo-finder-mount: Change the schema for finding repos on volumes

See issue #1174 for the rationale behind this. In summary:
 • It required two lists of collection–refs to be maintained: one in the
   repository, and one pointing to the repository.
 • It didn’t automatically work for live USBs of OSs based on OSTree
   (where there’s always a repository at /ostree/repo).
 • It was unnecessarily complex.

The new scheme allows a list of repositories to be searched, but without
needing a layer of indirection through their collection–refs. It adds
/ostree/repo and /.ostree/repo as well-known repository locations which
are always checked on a mounted volume (if they exist).

Update the unit tests accordingly.

Signed-off-by: Philip Withnall <withnall@endlessm.com>
https://github.com/ostreedev/ostree/issues/1174

Closes: #1179
Approved by: cgwalters