Prohibit accessing nested classes/objects of class object using class literal

The fqname of class should be clear from code
Example: can't shorten A.Default.B.Default.C to A.B.C
Also fixes problem when nested class of enum class could be accessed via enum entry

compiler/testData/codegen/box/classes/kt249.kt

@@ -8,6 +8,6 @@ class Outer() {
 }
 
 fun box (): String {
-    val inner = Outer.Inner()
+    val inner = Outer.Default.Inner()
     return "OK"
 }