Only issue error, no unlinking

Unlinking proved to cause other problems in the typer, specifically if
typechecking members of non-existing package or things referring to the
unlinked package

Author: felixmulder

src/dotty/tools/dotc/reporting/diagnostic/messages.scala

@@ -895,7 +895,7 @@ object messages {
 
   case class PkgDuplicateSymbol(existing: Symbol)(implicit ctx: Context)
   extends Message(33) {
-    val kind = "Duplicate Type"
+    val kind = "Duplicate Symbol"
     val msg = hl"trying to define package with same name as `$existing`"
     val explanation = ""
   }

src/dotty/tools/dotc/typer/Namer.scala

@@ -347,14 +347,13 @@ class Namer { typer: Typer =>
 
     if ((existing is Package) && (pkgOwner eq existing.owner)) existing
     else {
-      // If the name exists as type we should unlink the existing name from the
-      // scope, issue an error and continue, as usual:
+      /** If there's already an existing type, then the package is a dup of this type */
       val existingTpe = pkgOwner.info.decls.lookup(pid.name.toTypeName)
       if (existingTpe != NoSymbol) {
         ctx.error(PkgDuplicateSymbol(existingTpe), pid.pos)
-        pkgOwner.info.decls.openForMutations.unlink(existingTpe)
+        ctx.newCompletePackageSymbol(pkgOwner, (pid.name.asTermName.show + "$$package").toTermName).entered
       }
-      ctx.newCompletePackageSymbol(pkgOwner, pid.name.asTermName).entered
+      else ctx.newCompletePackageSymbol(pkgOwner, pid.name.asTermName).entered
     }
   }
 
@@ -388,9 +387,10 @@ class Namer { typer: Typer =>
     localCtx
   }
 
-   /** For all class definitions `stat` in `xstats`: If the companion class if not also defined
-   *  in `xstats`, invalidate it by setting its info to NoType.
-   */
+  /** For all class definitions `stat` in `xstats`: If the companion class if
+    * not also defined in `xstats`, invalidate it by setting its info to
+    * NoType.
+    */
   def invalidateCompanions(pkg: Symbol, xstats: List[untpd.Tree])(implicit ctx: Context): Unit = {
     val definedNames = xstats collect { case stat: NameTree => stat.name }
     def invalidate(name: TypeName) =