Comment 2 for bug 1873713

I didn't include a specific package because I didn't know how to find it (I ran apt and got the error I posted, but I wasn not installing one package by myself, instead it was part of the upgrade to Ubuntu 20.04).

I finally solved this bug by a series of:
apt upgrade
apt dist-upgrade
...
apt upgrade
apt dist-upgrade

Each one was failing for a different reason, but doing some progress as each time the list of changes was smaller. Finally they converged and my system got up to date.