Add support for nix packages

We should add support for https://nixos.org/ packages
An example of a nix "derivation" is at https://github.com/nix-community/nix-direnv/blob/master/flake.nix
This is like a package manifest.
For instance, this is one of the packages of ScanCode in nix:
https://github.com/NixOS/nixpkgs/blob/nixos-21.11/pkgs/development/python-modules/pyahocorasick/default.nix#L29

Of note: it uses its own "language" for license declaration. We may need a simple pygmars-based nix parser. Check also https://framagit.org/upt/upt-nix/-/blob/master/upt_nix/upt_nix.py

Some other pointers include:
- https://github.com/pombredanne/nix-derivation-tool