Fix bootstrapping for publishLocal by specifying the play json dependency

Author: abeln

project/Build.scala

@@ -537,7 +537,7 @@ object Build {
       // get libraries onboard
       libraryDependencies ++= Seq(
         "org.scala-lang.modules" % "scala-asm" % "6.0.0-scala-1", // used by the backend
-        "com.typesafe.play" %% "play-json" % "2.6.10", // used by explicit-nulls 
+        "com.typesafe.play" % "play-json_2.12" % "2.6.10", // used by explicit-nulls 
         // FIXME: Not needed, but should be on the compiler CP
         ("org.scala-lang.modules" %% "scala-xml" % "1.1.0").withDottyCompat(scalaVersion.value),
         "org.scala-lang" % "scala-library" % scalacVersion % "test",