refactor(cdk/scrolling): remove circular dependencies

Uses type-only imports to resolve the circular dependencies in the scrolling module.

Author: crisbeto

goldens/ts-circular-deps.json

@@ -1,10 +1 @@
-[
-  [
-    "src/cdk/scrolling/scroll-dispatcher.ts",
-    "src/cdk/scrolling/scrollable.ts"
-  ],
-  [
-    "src/cdk/scrolling/virtual-scroll-strategy.ts",
-    "src/cdk/scrolling/virtual-scroll-viewport.ts"
-  ]
-]
+[]

src/cdk/scrolling/scroll-dispatcher.ts

@@ -11,7 +11,7 @@ import {Platform} from '@angular/cdk/platform';
 import {ElementRef, Injectable, NgZone, OnDestroy, Optional, Inject} from '@angular/core';
 import {fromEvent, of as observableOf, Subject, Subscription, Observable, Observer} from 'rxjs';
 import {auditTime, filter} from 'rxjs/operators';
-import {CdkScrollable} from './scrollable';
+import type {CdkScrollable} from './scrollable';
 import {DOCUMENT} from '@angular/common';
 
 /** Time in ms to throttle the scrolling events by default. */

src/cdk/scrolling/virtual-scroll-strategy.ts

@@ -8,7 +8,7 @@
 
 import {InjectionToken} from '@angular/core';
 import {Observable} from 'rxjs';
-import {CdkVirtualScrollViewport} from './virtual-scroll-viewport';
+import type {CdkVirtualScrollViewport} from './virtual-scroll-viewport';
 
 /** The injection token used to specify the virtual scrolling strategy. */
 export const VIRTUAL_SCROLL_STRATEGY = new InjectionToken<VirtualScrollStrategy>(