PY-28341 Already used entries don't removed from completion list

Entries that were used in code remain in the completion list.
Return type of PyQualifiedReference updated to reflect the fact that it
returns PyTargetExpression's only.

python/testSrc/com/jetbrains/python/PythonCompletionTest.java

@@ -1335,6 +1335,11 @@ public class PythonCompletionTest extends PyTestCase {
     runWithLanguageLevel(LanguageLevel.PYTHON34, this::assertSingleVariantInExtendedCompletion);
   }
 
+  // PY-28341
+  public void testCompletionForUsedAttribute() {
+    doMultiFileTest();
+  }
+
   // PY-28103
   public void testPrintFunctionWithoutFuture() {
     final List<String> suggested = doTestByText("pr<caret>");