packfile: refactor `get_multi_pack_index()` to work on sources

packfile: refactor `get_multi_pack_index()` to work on sources

The function `get_multi_pack_index()` loads multi-pack indices via
`prepare_packed_git()` and then returns the linked list of multi-pack
indices that is stored in `struct object_database`. That list is in the
process of being removed though in favor of storing the MIDX as part of
the object database source it belongs to.

Refactor `get_multi_pack_index()` so that it returns the multi-pack
index for a single object source. Callers are now expected to call this
function for each source they are interested in. This requires them to
iterate through alternates, so we have to prepare alternate object
sources before doing so.

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