packfile: only prepare owning store in `packfile_store_prepare()`

packfile: only prepare owning store in `packfile_store_prepare()`

When calling `packfile_store_prepare()` we prepare not only the provided
packfile store, but also all those of all other sources part of the same
object database. This was required when the store was still sitting on
the object database level. But now that it sits on the source level it's
not anymore.

Refactor the code so that we only prepare the single packfile store
passed by the caller. Adapt callers accordingly.

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