+build Add sort imports support. (#28780)

* Add scalafix plugin for jdk 9.
* Add command alias sortImports.
* Excludes some sources from SortImports.
* Update SortImports to 0.4.0
* Sort imports with `sortImports` command.

project/ScalaFixExtraRulesPlugin.scala

@@ -0,0 +1,20 @@
+/*
+ * Copyright (C) 2020 Lightbend Inc. <https://www.lightbend.com>
+ */
+
+package akka
+
+import sbt.{AutoPlugin, PluginTrigger, Plugins, ScalafixSupport}
+import scalafix.sbt.ScalafixPlugin
+
+object ScalaFixExtraRulesPlugin extends AutoPlugin with ScalafixSupport{
+  override def trigger: PluginTrigger = allRequirements
+
+  override def requires: Plugins = ScalafixPlugin
+
+  import sbt._
+  import scalafix.sbt.ScalafixPlugin.autoImport.scalafixDependencies
+  override def projectSettings: Seq[Def.Setting[_]] = super.projectSettings ++ {
+    scalafixDependencies in ThisBuild += "com.nequissimus" %% "sort-imports" % "0.4.0"
+  }
+}