packfile: move get_multi_pack_index() into "midx.c"

The `get_multi_pack_index()` function is declared and implemented in the
packfile subsystem, even though it really belongs into the multi-pack
index subsystem. The reason for this is likely that it needs to call
`packfile_store_prepare()`, which is not exposed by the packfile system.
In a subsequent commit we're about to add another caller outside of the
packfile system though, so we'll have to expose the function anyway.

Do so now already and move `get_multi_pack_index()` into the MIDX
subsystem.

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

midx.c

@@ -93,6 +93,12 @@ static int midx_read_object_offsets(const unsigned char *chunk_start,
 	return 0;
 }
 
+struct multi_pack_index *get_multi_pack_index(struct odb_source *source)
+{
+	packfile_store_prepare(source->odb->packfiles);
+	return source->midx;
+}
+
 static struct multi_pack_index *load_multi_pack_index_one(struct odb_source *source,
 							  const char *midx_name)
 {