Make Scala-platform API independent of Java namespaces

Modularisation of the platform is not at odds with this suggestion. In fact, as the platform is modularised it makes even less sense to have dependencies on the Java libraries: If pulling in a scala.io module would add a transitive dependency on artifacts in java.io—which is today’s status quo—those transitive dependencies would also have to be covered by separate façades for other platforms, making successful cross compilation less predictable.