pgraph: Make our slow toposort even slower

I think this makes it more deterministic, but I'm not sure it matters,
since we are comparing based in the .String() property, and some nodes
have the same value, so it ends up depending on the order they're added
to the graph datastructure, but then we lose this information since it's
a map. Yuck.

pgraph/pgraph.go

@@ -707,8 +707,13 @@ func (g *Graph) DeterministicTopologicalSort() ([]Vertex, error) { // kahn's alg
 		v := S[last]
 		S = S[:last]
 		L = append(L, v) // add v to tail of L
-		// This doesn't need to loop in a deterministically sorted order.
+
+		var vertices []Vertex
 		for n := range g.adjacency[v] { // map[Vertex]Edge
+			vertices = append(vertices, n)
+		}
+		sort.Sort(VertexSlice(vertices)) // add determinism
+		for _, n := range vertices {     // map[Vertex]Edge
 			// for each node n remaining in the graph, consume from
 			// remaining, so for remaining[n] > 0
 			if remaining[n] > 0 {