packfile: move packed source into "odb/" subsystem

packfile: move packed source into "odb/" subsystem

In subsequent patches we'll be turning `struct odb_source_packed` into a
proper `struct odb_source`. As a first step towards this goal, move its
struct out of "packfile.{c,h}" and into "odb/source-packed.{c,h}".

This detaches the implementation of the packfile object source from the
generic packfile code, following the same convention already used by the
"files" and "in-memory" sources.

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