ensure that graph attributes are not lost

- this entails making Module.isSealed==true if attributes are set
- also removed Module.nest(), which implied fixing replaceShape to form
  a CompositeModule where CopiedModule was used before (GraphModule and
  TlsModule)

akka-stream-tests/src/test/scala/akka/stream/impl/fusing/InterpreterSpec.scala

@@ -511,7 +511,9 @@ class InterpreterSpec extends AkkaSpec with GraphInterpreterSpecKit {
       downstream.requestOne()
       lastEvents() should be(Set(RequestOne))
 
-      upstream.onComplete()
+      EventFilter[IllegalArgumentException](pattern = ".*Cannot pull closed port.*", occurrences = 1).intercept {
+        upstream.onComplete()
+      }
       val ev = lastEvents()
       ev.nonEmpty should be(true)
       ev.forall {