builtin/pack-objects: use `packfile_store_for_each_object()`

builtin/pack-objects: use `packfile_store_for_each_object()`

When enumerating objects that are supposed to be stored in a new cruft
pack we use `for_each_packed_object()` and then derive each object's
mtime individually. Refactor this logic to instead use the new
`packfile_store_for_each_object()` function with an object info request
that asks for the respective mtimes.

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