object-file: force objects loose via generic interface

object-file: force objects loose via generic interface

When repacking objects we may end up "loosening" objects via
`force_objects_loose()`. The implementation of this logic still sits
with "object-file.c" even though it is ultimately an implementation
detail of the "files" backend.

Moving this logic around is non-trivial though as we depend on
`write_loose_object()`, which is an internal implementation detail of
how we write loose objects. Until now it wasn't possible to use the
generic function `odb_source_write_object()` though, because the "loose"
implementation thereof would skip writing the object in case it already
exists in any other source.

This restriction was lifted over the preceding commits though, where
this object existence check is now handled on the object database level
and not on the individual source level anymore. Consequently, it is now
possible to use generic interfaces.

Refactor the code accordingly so that we can move the logic around in a
subsequent commit.

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