All cdkDrag items in a CdkDrop element are assumed to be members of a reorderable list

Currently all `cdkDrag` items in a `CdkDrop` element are treated as reorderable items in a list. While the underlying data structure is still left to the developer, the `CdkDragDrop` system updates the UI temporarily with changes to the "list order" even if the items are not supposed to be treated as a list.

Instead, we should have two directives for `CdkDrop` areas, `cdkDropZone` and `cdkDropList`. The former, does not do any automatic UI changes during the drag action and the latter does the current action of reordering items in the list.

首先跟踪 issue 中描述的 CdkDrop、CdkDragDrop、cdkDropZone 和 cdkDropList 入口点。确定当前的 drop 行为如何更新列表顺序，然后定义分离的行为，使 cdkDropZone 保持 UI 顺序不变，而 cdkDropList 保留重新排序行为。

由索引模型根据 Issue 内容生成。