7ec343309b got us most of the way,
but the last mile was botched by having the pending calls in the wrong
order as this way we potentially 'force' dpkg to remove/purge a package
it doesn't want to as another package still depends on it and the
replacement isn't fully installed yet.
So what we do now is a configure before remove and purge (all with
--no-triggers) and finishing off with another configure pending call to
take care of the triggers.
Note that in the bugreport example our current planner is forcing dpkg
to remove the package earlier via --force-depends which we could do for
the pending calls as well and could be used as a workaround, but we want
to do less forcing eventually.