odb/source-packed: wire up `freshen_object()` callback

odb/source-packed: wire up `freshen_object()` callback

Move `packfile_store_freshen_object()` and from "packfile.c" into
"odb/source-packed.c" and wire it up as the `freshen_object()` callback
of the "packed" source.

Note that this removes the last external caller of `find_pack_entry()`
from "packfile.c", which means that we can now make this function
static.

Signed-off-by: Patrick Steinhardt <redacted>
Signed-off-by: Junio C Hamano <redacted>