dissect: allow confext/sysext to be in the same image

dissect: allow confext/sysext to be in the same image

This reworks the image discovery logic, and conceptually allows DDIs
that are both confext and sysext to exist. Previously we'd only extract
one type of exension data from a DDI, with this we allow to extract both
if both exist.

This doesn't add support for true "multi-modal" DDIs, that qualify as
various things at once, it just lays some ground work that ensures we at
least can dissect such images.

This reworks 484d26dac1e8e543fc9e300e3c1fa36be0769f7d quite a bit.

This changes systemd-dissect's JSON output, but given the
version with the fields it changes/dops has never been released (as the
above patch was merged post-v254) this shouldn't be an issue.