We now track target packages and only require constraints on those
targets to be satisfiable. This allows us to overconstrain packages
that we do not care about, which is useful for excluding broken
packages.
We also now have a more general way of specifying constraints.
Previously constraints were specified as the conjunction of a version
range predicate and an optional installed constraint. This form made
it impossible to express constraints such as "exclude this source
package". Constraints for a package name are now specified simply by
a function predicate on the package version and installed/source state.