Parallelize planning

[ejholmes]

walk/plan.go

Lines 128 to 134 in 11c4e90

	// TODO(ejholmes): Accept a semaphore and parallelize this. No
	// need to perform this serially.
	dep, err := p.newTarget(ctx, d)
	if err != nil {
	return err
	}
	p.graph.Connect(t, dep)