Generalize member computation for package members

Previously, package-object local definitions won over global
ones. It's more systematic to form the union of both.

Also, absent dummy modules or classes have to be ignored in order
not to poison the union.

These changes would have made sense also for old-style package objects, btw.

Author: odersky

compiler/src/dotty/tools/dotc/core/SymDenotations.scala

@@ -1991,7 +1991,9 @@ object SymDenotations {
           if (pcls.isCompleting) recur(pobjs1, acc)
           else recur(pobjs1, acc.union(pcls.computeNPMembersNamed(name)))
         case nil =>
-          if (acc.exists) acc else super.computeNPMembersNamed(name)
+          val directMembers = super.computeNPMembersNamed(name)
+          if (acc.exists) acc.union(directMembers.filterWithPredicate(!_.symbol.isAbsent))
+          else directMembers
       }
       if (symbol `eq` defn.ScalaPackageClass) {
         val denots = super.computeNPMembersNamed(name)