dotty phase 2: scalafix ExplicitNonNullaryApply (#28949)

* scalafix ExplicitNonNullaryApply prepare

+ Temporarily use com.sandinh:sbt-scalafix because scalacenter/scalafix#1098
+ Add ExplicitNonNullaryApply rule to .scalafix.conf
+ Manually fix a NonNullaryApply case in DeathWatchSpec that cause
  `fixall` fail because ExplicitNonNullaryApply rule incorrectly rewrite
  `context unbecome` to `context unbecome()` instead of `context.unbecome()`

* scalafix ExplicitNonNullaryApply

fix by enabling only ExplicitNonNullaryApply rule in .scalafix.conf then:
```
% sbt -Dakka.build.scalaVersion=2.13.1
> fixall
```

* scalafmtAll

* Revert to ch.epfl.scala:sbt-scalafix

Co-authored-by: Bùi Việt Thành <thanhbv@sandinh.net>

akka-stream/src/main/scala/akka/stream/impl/fusing/FlatMapPrefix.scala

@@ -27,7 +27,7 @@ import scala.util.control.NonFatal
   override def initialAttributes: Attributes = DefaultAttributes.flatMapPrefix
 
   override def createLogicAndMaterializedValue(inheritedAttributes: Attributes): (GraphStageLogic, Future[M]) = {
-    val matPromise = Promise[M]
+    val matPromise = Promise[M]()
     val logic = new GraphStageLogic(shape) with InHandler with OutHandler {
       val accumulated = collection.mutable.Buffer.empty[In]