object-name: move logic to iterate through loose prefixed objects

object-name: move logic to iterate through loose prefixed objects

The logic to iterate through loose objects that have a certain prefix is
currently hosted in "object-name.c". This logic reaches into specifics
of the loose object source, so it breaks once a different backend is
used for the object storage.

Move the logic to iterate through loose objects with a prefix into
"object-file.c". This is done by extending the for-each-object options
to support an optional prefix that is then honored by the loose source.
Naturally, we'll also have this support in the packfile store. This is
done in the next commit.

Furthermore, there are no users of the loose cache outside of
"object-file.c" anymore. As such, convert `odb_source_loose_cache()` to
have file scope.

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