All package refs should use TERMREFpkg for pickling

Previously, some references used TermRef and a name with signature,
which gave  two different ways to represent a package.

Author: odersky

src/dotty/tools/dotc/core/pickling/TreePickler.scala

@@ -171,8 +171,11 @@ class TreePickler(pickler: TastyPickler) {
           else pickleRef()
         }
       case tpe: TermRefWithSignature =>
-        writeByte(TERMREF)
-        pickleNameAndSig(tpe.name, tpe.signature); pickleType(tpe.prefix)
+        if (tpe.symbol.is(Flags.Package)) picklePackageRef(tpe.symbol)
+        else {
+          writeByte(TERMREF)
+          pickleNameAndSig(tpe.name, tpe.signature); pickleType(tpe.prefix)
+        }
       case tpe: NamedType =>
         if (tpe.name == tpnme.Apply && tpe.prefix.argInfos.nonEmpty && tpe.prefix.isInstantiatedLambda)
           // instantiated lambdas are pickled as APPLIEDTYPE; #Apply will
@@ -187,10 +190,8 @@ class TreePickler(pickler: TastyPickler) {
           pickleName(tpe.name); pickleType(tpe.prefix)
         }
       case tpe: ThisType =>
-        if (tpe.cls.is(Flags.Package) && !tpe.cls.isEffectiveRoot) {
-          writeByte(TERMREFpkg)
-          pickleName(qualifiedName(tpe.cls))
-        }
+        if (tpe.cls.is(Flags.Package) && !tpe.cls.isEffectiveRoot) 
+          picklePackageRef(tpe.cls)
         else {
           writeByte(THIS)
           pickleType(tpe.tref)
@@ -259,6 +260,11 @@ class TreePickler(pickler: TastyPickler) {
         println(i"error while pickling type $tpe")
         throw ex
     }
+    
+    def picklePackageRef(pkg: Symbol): Unit = {
+      writeByte(TERMREFpkg)
+      pickleName(qualifiedName(pkg))
+    }
 
     def pickleMethodic(result: Type, names: List[Name], types: List[Type]) =
       withLength {