packfile: move `get_multi_pack_index()` into "midx.c"

packfile: move `get_multi_pack_index()` into "midx.c"

The `get_multi_pack_index()` function is declared and implemented in the
packfile subsystem, even though it really belongs into the multi-pack
index subsystem. The reason for this is likely that it needs to call
`packfile_store_prepare()`, which is not exposed by the packfile system.
In a subsequent commit we're about to add another caller outside of the
packfile system though, so we'll have to expose the function anyway.

Do so now already and move `get_multi_pack_index()` into the MIDX
subsystem.

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