scala/src/META-INF/scala-contents.xml

<idea-plugin>
  <extensions defaultExtensionNs="com.google.idea.blaze">
    <SyncPlugin implementation="com.google.idea.blaze.scala.sync.BlazeScalaSyncPlugin"/>
    <JavaLikeLanguage
        implementation="com.google.idea.blaze.scala.sync.source.ScalaJavaLikeLanguage"/>
  </extensions>
</idea-plugin>