odb: move MRU list of packfiles into `struct packfile_store`

odb: move MRU list of packfiles into `struct packfile_store`

The object database tracks the list of packfiles in most-recently-used
order, which is mostly used to favor reading from packfiles that contain
most of the objects that we're currently accessing. With the
introduction of the `struct packfile_store` we have a better place to
host this list though.

Move the list accordingly.

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