lib/repo: Change resolve_keyring_for_collection() to return a remote

Instead of returning just the keyring filename, return the entire
OstreeRemote, which has the keyring filename as one of its members. This
will simplify some upcoming changes, and allows slightly improved debug
logging.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Closes: #1202
Approved by: cgwalters

src/libostree/ostree-repo.h

@@ -1208,10 +1208,10 @@ gboolean ostree_repo_pull_from_remotes_finish (OstreeRepo    *self,
                                                GError       **error);
 
 _OSTREE_PUBLIC
-gchar *ostree_repo_resolve_keyring_for_collection (OstreeRepo    *self,
-                                                   const gchar   *collection_id,
-                                                   GCancellable  *cancellable,
-                                                   GError       **error);
+OstreeRemote *ostree_repo_resolve_keyring_for_collection (OstreeRepo    *self,
+                                                          const gchar   *collection_id,
+                                                          GCancellable  *cancellable,
+                                                          GError       **error);
 
 _OSTREE_PUBLIC
 gboolean ostree_repo_list_collection_refs (OstreeRepo                 *self,