nix: poetry2nix -> buildPythonPackage

The goal of this change is to enable users to use the devShell that nxc's flake provides without the need of duplicating Python (!) and many Python packages.

Main con: in the future, poetry shell and nix develop can provide different environments if we are not careful about updating poetry.lock after a nixpkgs update.

This PR include various related changes.

• use a nixpkgs tag rather than a channel (reduce maintenance pain)
• add nur-kapack as flake input (needed for execo)
• filter sources to avoid useless rebuilds
• remove outside tools from this repo
• relax click version constraint
• remove the init command (looked outdated/invalid)
• update poetry.lock such that it uses the versions defined by the nix flake