Drag and drop does not work correctly with horizontal block lists

[talldan]

Describe the bug
This is something that I only discovered by looking at the code, but Drag and Drop only works correctly with vertical block lists.

The useBlockDropZone hook only uses the y position for determining the drop position:
https://github.com/WordPress/gutenberg/blob/master/packages/block-editor/src/components/use-block-drop-zone/index.js#L184-L206.

Often the block ends up in the wrong place and the drop zone is highlighted incorrectly.

Seems like this was probably an accidental oversight when introducing horizontal block lists.

To reproduce
Steps to reproduce the behavior:

1. Add a buttons block and add multiple buttons
2. Try to reorder them using drag and drop
3. Observe the behavior is unpredictable and

Expected behavior
Drag and dropping works consistently in horizontal block lists:

• Blocks drop in the correct location
• Drop zones are indicated correctly using vertial lines/border.