for installation of requirements in RequirementSet.

        The returned list contains a requirement before another that depends on
        it. This helps ensure that the environment is kept consistent as they
        get installed one-by-one.

        The current implementation creates a topological ordering of the
        dependency graph, giving more weight to packages with less
        or no dependencies, while breaking any cycles in the graph at
        arbitrary points. We make no guarantees about where the cycle
        would be broken, other than it *would* be broken.
        Nz