[Python][Annotations] Should structure of annotations reflect structure of PyArrow at runtime

### Describe the enhancement requested

Currently proposed PyArrow annotations are stored in files to reflecting module names. PyArrow at runtime is structured slightly differently and the difference is adjusted for with annotation imports. As [discussed here](https://github.com/astral-sh/docstring-adder/issues/184) we should probably structure .pyi files such that they reflect the runtime structure of PyArrow.

Alternatively we could copy annotations into appropriately named files at wheel build time.

### Component(s)

Python

Start by comparing the proposed .pyi file layout with PyArrow's runtime module structure and tracing the annotation imports that bridge the difference. Decide whether the files should mirror the runtime structure or be copied into appropriately named files during wheel builds; done means the chosen layout works without those structural adjustments.

Written by the indexing model from the issue text.