packfile: refactor `find_pack_entry()` to work on the packfile store

packfile: refactor `find_pack_entry()` to work on the packfile store

The function `find_pack_entry()` doesn't work on a specific packfile
store, but instead works on the whole repository. This causes a bit of a
conceptual mismatch in its callers:

  - `packfile_store_freshen_object()` supposedly acts on a store, and
    its callers know to iterate through all sources already.

  - `packfile_store_read_object_info()` behaves likewise.

The only exception that doesn't know to handle iteration through sources
is `has_object_pack()`, but that function is trivial to adapt.

Refactor the code so that `find_pack_entry()` works on the packfile
store level instead.

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