KT-6122 Code completion in when in enum extension methods suggests non-qualified enum entry names

#KT-6122 Fixed

idea/testData/completion/basic/common/NoNestedClassesFromImplicitQualifier.kt

@@ -0,0 +1,11 @@
+class Outer {
+    class Nested
+    inner class Inner
+}
+
+fun Outer.foo() {
+    <caret>
+}
+
+// ABSENT: Nested
+// ABSENT: Inner