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

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

Move `packfile_store_find_abbrev_len()` and its associated helpers from
"packfile.c" into "odb/source-packed.c" and wire it up as the
`find_abbrev_len()` callback of the "packed" source.

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