Immutable scala sets don't work with extern annotation in Scala 2

[andreagilot-da]

When verifying the following snippet in stainless-scalac:

import stainless.lang._
import stainless.annotation._
import scala.collection.immutable.{Set => ScalaSet}

case class Set[T] (@extern s: ScalaSet[T])

Stainless won't accept it although there is an extern annotation.

[ Error  ] utils/AxiomaticSet.scala:8:20: Scala API `scala.collection.immutable.Set[T]` is not directly supported, please use `stainless.lang.Set` instead.
           case class Set[T] (@extern s: ScalaSet[T])
                              ^^^^^^^^^^^^^^^^^^^^^^

The same snippet works fine if we change immutable scala sets by mutable ones.