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

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

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

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