packfile: stop using linked MIDX list in `get_all_packs()`

packfile: stop using linked MIDX list in `get_all_packs()`

Refactor `get_all_packs()` so that we stop using the linked list of
multi-pack indices. Note that there is no need to explicitly prepare
alternates, and neither do we have to use `get_multi_pack_index()`,
because `prepare_packed_git()` already takes care of populating all data
structures for us.

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