Upstream formatter change in https://github.com/NixOS/nixpkgs/pull/512319 caused tests to fail.
Reformat repository using new nixfmt-rfc-style.
Instead of having to manually stub packages that should not be downloaded we instead automatically stub all packages (except a small list of whitelisted ones). Tests can re-introduce the real package by using the `realPkgs` module argument.
ruff is Python linter and code formatter, written in Rust. See <https://docs.astral.sh/ruff/>.
