refactor(cdk-experimental/combobox): switch to standalone

Reworks `cdk-experimental/combobox` to support standalone.

Author: crisbeto

src/cdk-experimental/combobox/combobox-module.ts

@@ -13,8 +13,7 @@ import {CdkComboboxPopup} from './combobox-popup';
 
 const EXPORTED_DECLARATIONS = [CdkCombobox, CdkComboboxPopup];
 @NgModule({
-  imports: [OverlayModule],
+  imports: [OverlayModule, ...EXPORTED_DECLARATIONS],
   exports: EXPORTED_DECLARATIONS,
-  declarations: EXPORTED_DECLARATIONS,
 })
 export class CdkComboboxModule {}

src/cdk-experimental/combobox/combobox-popup.ts

@@ -21,6 +21,7 @@ let nextId = 0;
     'tabindex': '-1',
     '(focus)': 'focusFirstElement()',
   },
+  standalone: true,
 })
 export class CdkComboboxPopup<T = unknown> implements OnInit {
   @Input()

src/cdk-experimental/combobox/combobox.spec.ts

@@ -3,7 +3,7 @@ import {ComponentFixture, TestBed, waitForAsync} from '@angular/core/testing';
 import {By} from '@angular/platform-browser';
 import {CdkComboboxModule} from './combobox-module';
 import {CdkCombobox} from './combobox';
-import {dispatchKeyboardEvent, dispatchMouseEvent} from '../../cdk/testing/private';
+import {dispatchKeyboardEvent, dispatchMouseEvent} from '@angular/cdk/testing/private';
 import {DOWN_ARROW, ESCAPE} from '@angular/cdk/keycodes';
 import {CdkComboboxPopup} from '@angular/cdk-experimental/combobox/combobox-popup';
 
@@ -25,8 +25,7 @@ describe('Combobox', () => {
 
     beforeEach(waitForAsync(() => {
       TestBed.configureTestingModule({
-        imports: [CdkComboboxModule],
-        declarations: [ComboboxToggle],
+        imports: [CdkComboboxModule, ComboboxToggle],
       }).compileComponents();
     }));
 
@@ -189,8 +188,7 @@ describe('Combobox', () => {
 
     beforeEach(waitForAsync(() => {
       TestBed.configureTestingModule({
-        imports: [CdkComboboxModule],
-        declarations: [ComboboxToggle],
+        imports: [CdkComboboxModule, ComboboxToggle],
       }).compileComponents();
     }));
 
@@ -260,8 +258,7 @@ describe('Combobox', () => {
 
     beforeEach(waitForAsync(() => {
       TestBed.configureTestingModule({
-        imports: [CdkComboboxModule],
-        declarations: [ComboboxToggle],
+        imports: [CdkComboboxModule, ComboboxToggle],
       }).compileComponents();
     }));
 
@@ -385,6 +382,8 @@ describe('Combobox', () => {
       <button id="applyButton" (click)="toggleCombobox.updateAndClose(input.value)">Apply</button>
     </div>
   </ng-template>`,
+  standalone: true,
+  imports: [CdkComboboxModule],
 })
 class ComboboxToggle {
   @ViewChild('input') inputElement: ElementRef<HTMLInputElement>;

src/cdk-experimental/combobox/combobox.ts

@@ -58,6 +58,7 @@ export const CDK_COMBOBOX = new InjectionToken<CdkCombobox>('CDK_COMBOBOX');
     '[attr.tabindex]': '_getTabIndex()',
   },
   providers: [{provide: CDK_COMBOBOX, useExisting: CdkCombobox}],
+  standalone: true,
 })
 export class CdkCombobox<T = unknown> implements OnDestroy {
   @Input('cdkComboboxTriggerFor')