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

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

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

As part of the move, `check_and_freshen_source()` is inlined into the
callback function, as it has no other callers anymore.

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