object-file: refactor writing objects to use loose source

object-file: refactor writing objects to use loose source

The "object-file" subsystem still hosts the majority of logic used to
write loose objects. Eventually, we'll want to move this logic into
"odb/source-loose.c", but this isn't yet easily possible because a lot
of the writing logic is still being shared with `force_object_loose()`.

We will eventually detangle this logic so that we can indeed move all of
it into the "loose" source. Meanwhile though, refactor the code so that
it operates on a `struct odb_source_loose` directly to already make the
dependency explicit.

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