// including clusters that are not present in the ancestors map. has_successors
// is the set of clusters that are ancestors of some other cluster.
void TopologicalClusterSort(
    const absl::flat_hash_set<string>& clusters,
    const absl::flat_hash_set<string>& has_successors,
    const absl::flat_hash_map<string, absl::flat_hash_set<string>>& ancestors,