object-file: move loose object map into loose source

object-file: move loose object map into loose source

The loose object map is used to map from the repository's canonical
object hash to the compatibility hash. As the name indicates, this map
is only used for loose objects, and as such it is tied to a specific
loose object source.

Same as with preceding commits, move this map into the loose object
source accordingly.

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