add AttachDependsOnTransformer to plan

This transformer is what will provider the data sources with the
transitive dependencies needed to determine if they can read during plan
or must be deferred.

terraform/graph_builder_plan.go

@@ -150,6 +150,10 @@ func (b *PlanGraphBuilder) Steps() []GraphTransformer {
 		// have to connect again later for providers and so on.
 		&ReferenceTransformer{},
 
+		// Make sure data sources are aware of any depends_on from the
+		// configuration
+		&AttachDependsOnTransformer{},
+
 		// Add the node to fix the state count boundaries
 		&CountBoundaryTransformer{
 			Config: b.Config,

terraform/transform_reference.go

@@ -163,7 +163,7 @@ func (t AttachDependsOnTransformer) Transform(g *Graph) error {
 			continue
 		}
 
-		// depMap will dedupe and only add resource references
+		// depMap will only add resource references and dedupe
 		m := make(depMap)
 
 		for _, dep := range refMap.DependsOn(v) {