packfile: create store via its owning source

packfile: create store via its owning source

In subsequent patches we're about to move the packfile store from the
object database layer into the object database source layer. Once done,
we'll have one packfile store per source, where the source is owning the
store.

Prepare for this future and refactor `packfile_store_new()` to be
initialized via an object database source instead of via the object
database itself.

This refactoring leads to a weird in-between state where the store is
owned by the object database but created via the source. But this makes
subsequent refactorings easier because we can now start to access the
owning source of a given store.

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