Homogenize skolem types

Skolem types are eliminated by pickling, so they should not appear
in the "before-pickling" output.

Author: odersky

src/dotty/tools/dotc/printing/PlainPrinter.scala

@@ -55,6 +55,8 @@ class PlainPrinter(_ctx: Context) extends Printer {
           if (tp1 eq tp) tp else homogenize(tp1)
         case tp: RefinedType =>
           tp.normalizeHkApply
+        case tp: SkolemType =>
+          homogenize(tp.info)
         case tp: LazyRef =>
           homogenize(tp.ref)
         case _ =>