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/io/IOSources.scala

@@ -41,6 +41,8 @@ private[akka] final class FileSource(f: File, chunkSize: Int, val attributes: At
 
   override def withAttributes(attr: Attributes): Module =
     new FileSource(f, chunkSize, attr, amendShape(attr))
+
+  override protected def label: String = s"FileSource($f, $chunkSize)"
 }
 
 /**