lib/repo-refs: Add ostree_repo_remote_list_collection_refs() API

lib/repo-refs: Add ostree_repo_remote_list_collection_refs() API

This parallels ostree_repo_remote_list_refs(), but returns a map of
OstreeCollectionRef → checksum, and includes refs from collection IDs
other than the remote repository’s main collection ID.

Use this in OstreeRepoFinderConfig to ensure that refs are matched
against even if they’re stored in the repository summary file’s
collection map, rather than its main ref map. This fixes false negatives
when searching for refs in some situations.

Signed-off-by: Philip Withnall <withnall@endlessm.com>
Closes: #1058
Approved by: cgwalters