odb: move list of packfiles into `struct packfile_store`

odb: move list of packfiles into `struct packfile_store`

The object database tracks the list of packfiles it currently knows
about. With the introduction of the `struct packfile_store` we have a
better place to host this list though.

Move the list accordingly. Extract the logic from `odb_clear()` that
knows to close all such packfiles and move it into the new subsystem, as
well.

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