packfile: introduce function to read object info from a store

packfile: introduce function to read object info from a store

Extract the logic to read object info for a packed object from
`do_oid_object_into_extended()` into a standalone function that operates
on the packfile store. This function will be used in a subsequent
commit.

Note that this change allows us to make `find_pack_entry()` an internal
implementation detail. As a consequence though we have to move around
`packfile_store_freshen_object()` so that it is defined after that
function.

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