refactor(cdk/drag-drop): move sorting and positioning logic into a separate class (#25044)

This is an internal reorganization of the drop list to make it easier to eventually add support for mixed orientation. Includes:
* Moving out all the sorting and positioning-related code out of `DropListRef` and into a separate class. The existing logic is very specific to our approach that moves items around using `transform`. The future mixed orientation will likely move the DOM nodes themselves which would've been difficult to manage if we kept the positioning logic within `DropListRef`.
* Moves some of the DOM-related utilities into the `dom` directory in order to reduce clutter in the root of the package.

Author: crisbeto

src/cdk/drag-drop/directives/drag.spec.ts

@@ -30,7 +30,7 @@ import {of as observableOf} from 'rxjs';
 import {DragDropModule} from '../drag-drop-module';
 import {CdkDragDrop, CdkDragEnter, CdkDragStart} from '../drag-events';
 import {Point, DragRef, PreviewContainer} from '../drag-ref';
-import {extendStyles} from '../drag-styling';
+import {extendStyles} from '../dom/styling';
 import {moveItemInArray} from '../drag-utils';
 
 import {CdkDrag} from './drag';

src/cdk/drag-drop/client-rect.ts renamed to src/cdk/drag-drop/dom/client-rect.ts

@@ -25,11 +25,11 @@ import {
   extendStyles,
   toggleNativeDragInteractions,
   toggleVisibility,
-} from './drag-styling';
-import {getTransformTransitionDurationInMs} from './transition-duration';
-import {getMutableClientRect, adjustClientRect} from './client-rect';
-import {ParentPositionTracker} from './parent-position-tracker';
-import {deepCloneNode} from './clone-node';
+} from './dom/styling';
+import {getTransformTransitionDurationInMs} from './dom/transition-duration';
+import {getMutableClientRect, adjustClientRect} from './dom/client-rect';
+import {ParentPositionTracker} from './dom/parent-position-tracker';
+import {deepCloneNode} from './dom/clone-node';
 
 /** Object that can be used to configure the behavior of DragRef. */
 export interface DragRefConfig {