packfile: thread odb_source_packed through packed_object_info()

packfile: thread odb_source_packed through packed_object_info()

Add an optional `struct odb_source_packed *source` parameter to
`packed_object_info()` and `packed_object_info_with_index_pos()`. This
parameter is unused at this point in time, but it will be used in a
follow-up commit so that we can record the source of a specific object.

Note that callers in "odb/source-packed.c" pass the already-available
source, but all other callers pass `NULL` instead. This is fine though,
as we only care about populating this info when called via the packed
store.

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