simplify materialized value computation tree, fixes #20015

- also fixes materialized value sources for graphs that import zero or
  one graphs, with and without Fusing

akka-stream/src/main/scala/akka/stream/impl/FlowModule.scala

@@ -5,11 +5,12 @@ package akka.stream.impl
 
 import akka.stream._
 import akka.stream.impl.StreamLayout.Module
+import akka.event.Logging
 
 /**
  * INTERNAL API
  */
-private[stream] trait FlowModule[In, Out, Mat] extends StreamLayout.Module {
+private[stream] trait FlowModule[In, Out, Mat] extends StreamLayout.AtomicModule {
   override def replaceShape(s: Shape) =
     if (s != shape) throw new UnsupportedOperationException("cannot replace the shape of a FlowModule")
     else this
@@ -18,6 +19,6 @@ private[stream] trait FlowModule[In, Out, Mat] extends StreamLayout.Module {
   val outPort = Outlet[Out]("Flow.out")
   override val shape = new FlowShape(inPort, outPort)
 
-  override def subModules: Set[Module] = Set.empty
+  protected def label: String = Logging.simpleName(this)
+  final override def toString: String = f"$label [${System.identityHashCode(this)}%08x]"
 }
-