Include data files in editable builds (#19312)
## Summary
PEP 660 doesn't appear to be explicit about the treatment of `.data`
directories, beyond the following:
> With regard to the wheel .data directory, this PEP focuses on making
the purelib and platlib categories (installed into site-packages)
“editable”. It does not make special provision for the other categories
such as headers, data and scripts. Package authors are encouraged to use
console_scripts, make their scripts tiny wrappers around library
functionality, or manage these from the source checkout during
development.
Other build backends seem to snapshot them, which seems reasonable to
me.
Closes https://github.com/astral-sh/uv/issues/19258.
Co-authored-by: Codex <noreply@openai.com>
