Watch transitive dependencies for changes

[apaleslimghost]

From this comment.

There's a couple of ways we could go about this. Easiest would be to compile a module graph (module-deps?) and reload top-level modules when their descendants change.

Harder but requiring less reloading is to reload a module into its parent. We'd need to (somehow) work out what variable it's required into and use rewire to inject the change. I'm not sure if this is feasible.