Graphviz: assign all leaf modules max rank

[matejdro]

For example, for the following module graph:

digraph G {
":A" -> ":B"
":B" -> ":C"
":C" -> ":D"
":C" -> ":E"
":B" -> ":E"
":A" -> ":F"
":B" -> ":G"
}

It is not very easy to see which modules can be compiled concurrently, since leaves are scattered around the entire graph.

However, adding { rank=max; ":D"; ":E", ":G", ":F" } line to the generated graphviz makes this much better:

Now, we can clearly see that D, E, G and F will be compiled first, at the same time.

If accepted, I can add a PR for this.