lib/repo-finder: Add basic support for finding remote URIs by ref name

lib/repo-finder: Add basic support for finding remote URIs by ref name

Add an initial OstreeRepoFinder interface (but no implementations),
which will find remote URIs by ref names and collection IDs, the
combination of which is globally unique.

The new API is used in a new ostree_repo_find_updates() function, which
resolves a list of ref names to update into a set of remote URIs to pull
them from, which can be treated as mirrors. It is an attempt to
generalise resolution of the URIs to pull from, and to generalise
determination of the order and parallelisation which they should be
downloaded from in.

Includes fixes by Krzesimir Nowak <krzesimir@kinvolk.io>.

Signed-off-by: Philip Withnall <withnall@endlessm.com>
Closes: #924
Approved by: cgwalters