Refine typing of export forwarders

[odersky]

An export forwarder is now of singleton type only if the export path does not
refer to private values. Previously, this situation would lead to a type error
later on, like this one:

scala> class A{
     |   val x: Int = 1
     | }
     | class B(a: A){
     |   export a.x
     | }
5 |  export a.x
  |           ^
  |           non-private method x in class B refers to private value a
  |           in its type signature => (B.this.a.x : Int)