And add a few global package version pref policies and use them in
ordinary install and upgrade. For install we use a policy that says
that we prefer the latest version of a package that we specifically
request and prefer the installed version of any other package. For
upgrade we simple always prefer the latest version. One can imageine
other policies where we prefer the latest version for only some
interesting subset of packages and installed otherwise.
No resolvers actually make use of this preference yet.