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

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

Move the logic to prepare and reprepare the "packed" source into
"odb/source-packed.c" and wire it up as the `reprepare()` callback.

Note that "preparing" a source is not yet generic. Eventually, it would
probably make sense to turn the existing `reprepare()` callback into a
`prepare()` callback with an optional flag to force re-preparing. But
this step will be handled in a separate patch series.

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