In the bug, mutter was kept back due to phasing and the new gnome-shell
depended on that, and was therefore kept back as well, however,
gnome-shell-common was not broken, and apt decided to continue upgrading
it by removing gnome-shell and the ubuntu desktop meta packages.

This is potentially a regression of LP#1990586 where we added keep
back calls to the start of the dist-upgrade to ensure that we do not
mark stuff for upgrade in the first place that depends on phasing
updates, however it was generally allowed by the resolver to also
do those removals.

To fix this, we need to resolve the update normally and then use
ResolveByKeepInternal to keep back any changes broken by held back
packages.

However, doing so breaks test-bug-591882-conkeror because ResolveByKeep
keeps back packages for broken Recommends as well, which is not
something we generally want to do in a dist-upgrade after we already
decided to upgrade it.

To circumvent that issue, extend the pkgProblemResolver to allow
a package to be policy broken, and mark all packages that already
were already going to be policy broken to be allowed to be that,
such that we don't try to undo their installs.

LP: #2025462