object-file: move loose object cache into loose source

Our loose objects use a cache that (optionally) stores all objects for
each of the opened sharding directories. This cache is located in the
`struct odb_source`, but now that we have `struct odb_source_loose` it
makes sense to move it into the latter structure so that all state that
relates to loose objects is entirely self-contained.

Do so. While at it, rename corresponding functions to have a prefix that
relates to `struct odb_source_loose`.

Note that despite this prefix, the functions still accept a `struct
odb_source` as input. This is done intentionally: once we introduce
pluggable object databases, we will continue to accept this struct but
then do a cast inside these functions to `struct odb_source_loose`. This
design is similar to how we do it for our ref backends.

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

odb.c

@@ -370,7 +370,6 @@ static void odb_source_free(struct odb_source *source)
 {
 	free(source->path);
 	odb_source_loose_free(source->loose);
-	odb_clear_loose_cache(source);
 	loose_object_map_clear(&source->loose_map);
 	free(source);
 }