core: Make ostree_get_relative_object_path() private

I plan to rename all of these APIs to use the term 'loose', so that it
makes more sense after pack files are introduced.  External users
should not use them; instead use _load_variant() or _read_commit().

src/libostree/ostree-repo-pull.c

@@ -994,7 +994,7 @@ enqueue_one_object_request (OtPullData        *pull_data,
     }
   else
     {
-      objpath = ostree_get_relative_object_path (checksum, objtype, TRUE);
+      objpath = _ostree_get_relative_object_path (checksum, objtype, TRUE);
       obj_uri = suburi_new (pull_data->base_uri, objpath, NULL);
     }