Module dependencies

[jonascarpay]

Same as #4. There's already a --collapse-modules flag, but that's basically just a hack using the --collapse-* logic. I think it's time to add first-class support for this. Mermaid (#13) should be able to draw lines between modules in any graph. GraphViz doesn't allow the same kind of bigraph/subgraph structure, so we'd probably need to introduce a separate rendering mode.