build: Add hacks function for 2-way Nixpkgs Python infrastructure interop

[adisbladis]

This adds a hacks.toNixpkgs that generates a Nixpkgs overlay from pyproject.nix build (uv2nix) packages. It is conceptually similar to hacks.nixpkgsPrebuilt, just in reverse.

[ezemtsov]

@adisbladis thanks for the short talk at nixcon today!

Just not to have the results of our conversation lost, leaving a comment here.

Do you think it would be possible to setup hacks.toNixpkgs override to select all packages automatically from the uv lock file? It would be potentially easy to extract pname properties from pythonSet, but at this point it is mixed with many irrelevant attributes, such as pyprojectDistHook or pyprojectWheelHook.

Maybe we could move out hooks and other irrelevant attributes from the resulting pythonSet?

[adisbladis]

Do you think it would be possible to setup hacks.toNixpkgs override to select all packages automatically from the uv lock file? It would be potentially easy to extract pname properties from pythonSet, but at this point it is mixed with many irrelevant attributes, such as pyprojectDistHook or pyprojectWheelHook.

Yep! I've now implemented the packages arg in three different modes:

• null (default)
  Include all generated packages
• list
  Include explicitly listed packages
• predicate
  A predicate function that filters the list of attributes to generate