reduce module instances refs to the module call

There aren't going to be any nodes specifically for module call
instances during plan, so we have to switch the reference subject to the
general module call.

terraform/transform_reference.go

@@ -303,6 +303,8 @@ func (m ReferenceMap) References(v dag.Vertex) []dag.Vertex {
 				subject = ri.ContainingResource()
 			case addrs.AbsModuleCallOutput:
 				subject = ri.ModuleCallOutput()
+			case addrs.ModuleCallInstance:
+				subject = ri.Call
 			default:
 				log.Printf("[WARN] ReferenceTransformer: reference not found: %q", subject)
 				continue