setup: --prune-install appears to be broken

[David McFarland]

> You are misunderstanding what prune-install does: it ensures that only
> the exact list of groups/packages that you give it for installation
> (plus their dependencies) is installed when it finishes and you
> literally said you want to end up with no packages at all.

That's not what it's doing though. When run on an empty root, it
installs Base (the set of packages in category "Base"), and when run
again, it does nothing (i.e. leaves Base installed'), even if other
packages were installed.

I actually think what it _should_ do is leave Base installed, by
treating all Base packages as implicitly referenced:

      bool uninstall = (!(wanted  || base) && (deleted || PruneInstallOption))

So we uninstall packages if they are neither wanted nor Base, and either
explicitly deleted or --prune-install.

However, the bigger problem is that none of this is actually working,
because of that "Can't happen.  No packagemeta for base" error.