Source plugins typically do not have an accompanying YAML file with
their default settings.

Pulls will fail from the cache if there's no summary file, so we should
always create one.

If you need a cache that breaks when pulled from, pass an empty
directory that hasn't been initialized as a repo at all.

The cache parameters are now in highest-to-lowest precidence
order corresponding to left-to-right along the line where they are
specified.

Once we make it possible to specify a list of URLs for each of these
parameters, the precidence will continue to always read from highest
to lowest corresponding to left-to-right since each list of URLs
will also read left-to-right.

This call can fail (if we pass an invalid remote name, for example) so
we should wrap the GLib.Error with OSTreeException.

In the case of GLib.Error this is especially important as GLib.Error
can't be pickled and so such exceptions disappear completely when we try
to propagate them back from multiprocessing subprocesses. See
<https://bugzilla.gnome.org/show_bug.cgi?id=791265>.