packfile: extract function to iterate through objects of a store

packfile: extract function to iterate through objects of a store

In the next commit we're about to introduce a new function that knows to
iterate through objects of a given packfile store. Same as with the
equivalent function for loose objects, this new function will also be
agnostic of backends by using a `struct object_info`.

Prepare for this by extracting a new shared function to iterate through
a single packfile store.

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