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

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

Move `odb_source_loose_write_object()` from "object-file.c" into
"odb/source-loose.c" and wire it up as the `write_object()` callback of
the loose source.

As in preceding commits, this requires us to expose a couple of generic
functions from "object-file.c" as they are used in both subsystems now.

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