[FR] External Data Support

What's the problem this feature will solve?

Project Jupyter uses data_files for discovery of runtime extensions. This feature was deprecated without a viable alternative.

Describe the solution you'd like

The flit project recently added support for external_data, which is a simplified and constrained version of data_files. We could support an equivalent feature in setuptools, with caveats about when it is appropriate to use it.

Alternative Solutions

Project Jupyter and other current consumers of data_files could instead switch to using a flit backend, which we have been actively exploring.

Additional context

Project Jupyter uses data_files to allow libraries to provide shared setting and static assets files that are consumed by Jupyter at runtime. We have explored using entry_points for this purpose, but it did not scale well.

Code of Conduct

• I agree to follow the PSF Code of Conduct

Start with the linked flit external_data documentation and setuptools' existing data_files behavior described in the issue. Define the supported scope and caveats, then establish what equivalent support in setuptools would require and how it should be validated.