[NF] Fix expandable connector lookup.

- Only allow lookup of undeclared elements in components, not classes.

Belonging to [master]:
  - OpenModelica/OpenModelica#177
  - #3088

Author: perost

Compiler/NFFrontEnd/NFLookup.mo

@@ -778,6 +778,7 @@ algorithm
   try
     n := Class.lookupElement(name, cls);
   else
+    true := InstNode.isComponent(node);
     n := InstNode.NAME_NODE(name);
   end try;