Handle (t64) package replacements

We should do something like:

for package in cache:
  if package.is_installed and <candidate version is obsolete>:
     replacement = <find a package in the archive that Conflicts/Breaks + Replaces + Provides it>
     replacement.mark_install(auto_fix=False, auto_inst=False)
     replacement.mark_remove(auto_fix=False)

To hint the solver to remove the installed pre-t64 packages and install packages replacing it. We gotta do some auto_fix, auto_inst False stuff as we do not want to resolve the dependencies at this point, let the upgrader work on this.

I do not believe we have to protect our choice to remove the packages here like we did for gpg-wks-server as the packages are not updatable, but I can't say I've checked. Potentially maybe just the mark_install are enough since they will inevitably result in removals anyway, but this would be easier if the removal sticks around.