Multi-package stack projects

I'm usually working on a small universe of inter-related packages, all listed in the stack.yaml file. I'd love it if a single instance of stack-ide could keep up with transitive changes in that whole universe, just as stack does.

I guess this would entail going through the stack.yaml file and adding everything in `packages:` to the session? I could give this a shot — if someone has a pointer to where ide-backend determines which files to track that would be super helpful.