odb: move packfile map into `struct packfile_store`

odb: move packfile map into `struct packfile_store`

The object database tracks a map of packfiles by their respective paths,
which is used to figure out whether a given packfile has already been
loaded. With the introduction of the `struct packfile_store` we have a
better place to host this list though.

Move the map accordingly.

`pack_map_entry_cmp()` isn't used anywhere but in "packfile.c" anymore
after this change, so we convert it to a static function, as well. Note
that we also drop the `inline` hint: the function is used as a callback
function exclusively, and callbacks cannot be inlined.

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