object-file: get rid of the_repository in loose object iterators

The iterators for loose objects still rely on `the_repository`. Refactor
them:

  - `for_each_loose_file_in_objdir()` is refactored so that the caller
    is now expected to pass an `odb_source` as parameter instead of the
    path to that source. Furthermore, it is renamed accordingly to
    `for_each_loose_file_in_source()`.

  - `for_each_loose_object()` is refactored to take in an object
    database now and calls the above function in a loop.

This allows us to get rid of the global dependency.

Adjust callers accordingly.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/pack-objects.c

@@ -4342,9 +4342,8 @@ static int add_loose_object(const struct object_id *oid, const char *path,
  */
 static void add_unreachable_loose_objects(void)
 {
-	for_each_loose_file_in_objdir(repo_get_object_directory(the_repository),
-				      add_loose_object,
-				      NULL, NULL, NULL);
+	for_each_loose_file_in_source(the_repository->objects->sources,
+				      add_loose_object, NULL, NULL, NULL);
 }
 
 static int has_sha1_pack_kept_or_nonlocal(const struct object_id *oid)