odb/source-packed: extract logic to skip certain packs

odb/source-packed: extract logic to skip certain packs

The caller can pass flags that allow them to filter out specific kinds
of objects when iterating objects via `odb_for_each_object()`. This only
works for "normal" iteration though, as we `BUG()` when the user passes
flags and specifies an object prefix.

This limitation will be lifted in the next commit. Prepare for this by
extracting the logic that skips certain kinds of packs so that we can
easily reuse it.

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