odb/source-loose: wire up `for_each_object()` callback

odb/source-loose: wire up `for_each_object()` callback

Move `odb_source_loose_for_each_object()` and its associated helpers
from "object-file.c" into "odb/source-loose.c" and wire it up as the
`for_each_object()` callback of the loose source.

Again, as in the preceding commit, we are forced to expose a couple of
functions from "object-file.c" that are now used by both subsystems.

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