Merge branch 'ps/packfile-store-in-odb-source' into next

Merge branch 'ps/packfile-store-in-odb-source' into next

The packfile_store data structure is moved from object store to odb
source.

* ps/packfile-store-in-odb-source:
  packfile: move MIDX into packfile store
  packfile: refactor `find_pack_entry()` to work on the packfile store
  packfile: inline `find_kept_pack_entry()`
  packfile: only prepare owning store in `packfile_store_prepare()`
  packfile: only prepare owning store in `packfile_store_get_packs()`
  packfile: move packfile store into object source
  packfile: refactor misleading code when unusing pack windows
  packfile: refactor kept-pack cache to work with packfile stores
  packfile: pass source to `prepare_pack()`
  packfile: create store via its owning source