Consolidate labels widget architecture

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spec/frontend/vue_shared/components/sidebar/labels_select_widget/labels_select_root_spec.js

@@ -122,9 +122,9 @@ describe('LabelsSelectRoot', () => {
     });
   });
 
-  it('emits `updateSelectedLabels` event on dropdown contents `setLabels` event', async () => {
+  it('emits `updateSelectedLabels` event on dropdown contents `setLabels` event if iid is not set', async () => {
     const label = { id: 'gid://gitlab/ProjectLabel/1' };
-    createComponent();
+    createComponent({ config: { ...mockConfig, iid: undefined } });
 
     findDropdownContents().vm.$emit('setLabels', [label]);
     expect(wrapper.emitted('updateSelectedLabels')).toEqual([[[label]]]);