The general case in exploring the state space is that we have a set of
choices (package names) and for each choice we have a number of
versions of that package we could pick. If there's only one version of
a package then we make that choice first. Otherwise we have to pick
some package and select one of the available versions. The question is
which package should we make a choice for first? Previously we picked
completely arbitrarily. Surprisingly this actually works pretty well.
An improvement is to pick packages in topological order. This works
better because it allows dependencies from earlier choices to
constrain our later choices.