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

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

Move the logic to read object info from a "packed" source into
"odb/source-packed.c" and wire it up as the `read_object_info()`
callback.

Note that we also move around the supporting `find_pack_entry()`, but we
still have to expose it to other callers that exist in "packfile.c".
This will be fixed in subsequent commits though, where all callers in
"packfile.c" will have been moved into "odb/source-packed.c", and at
that point we'll be able to make `find_pack_entry()` file-local again.

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