Per-package preferences on the version. It is a soft constraint that the
DependencyResolver should try to respect where possible. It consists of
an InstalledPreference which says if we prefer versions of packages
that are already installed. It also has (possibly multiple)
PackageVersionPreferences which are suggested constraints on the version
number. The resolver should try to use package versions that satisfy
the maximum number of the suggested version constraints.
It is not specified if preferences on some packages are more important than others.