Add NIx support

[chinyeungli]

Reference: #201

The code tries to get package metadata from https://search.devbox.sh/v2/. If the package cannot be found there, it will try to use the nix command to retrieve the metadata (if nix is installed on the host system).

It then uses https://cache.nixos.org/ to retrieve the download url such as: https://cache.nixos.org/0kz6msxd62b2hx2pacs1vvl6al5ma3ri.narinfo

Note that there are certain requirements: a system qualifier and version are required.

In addition to getting the download URL of the binary, I've also implemented a function to get the upstream source download URL.

Some samples run:

>>> from fetchcode import fetch
>>> from fetchcode import nix
>>> f = fetch("pkg:nix/nixpkgs/vixl@8.0.0?system=aarch64-linux")
>>> f.url
'https://cache.nixos.org/nar/1p9ml56vj79vng8c91vjxfdjmydia19fr7lir40ns4hqcy7lac37.nar.zst'
>>> print(nix.Nix.get_upstream_src_download_url("pkg:nix/nixpkgs/vixl@8.0.0?system=aarch64-linux"))
https://github.com/Linaro/vixl/archive/8.0.0.tar.gz
>>> g = fetch("pkg:nix/nixpkgs/imlib2@1.12.4?system=x86_64-linux")
>>> g.url
'https://cache.nixos.org/nar/0jxici9pc89vqa656rwjzlvk1rwrl6zg9dn8gpvh3p8g62n48328.nar.xz'
>>> print(nix.Nix.get_upstream_src_download_url("pkg:nix/nixpkgs/imlib2@1.12.4?system=x86_64-linux"))
https://downloads.sourceforge.net/enlightenment/imlib2-1.12.4.tar.xz
>>> print(nix.Nix.get_upstream_src_download_url("pkg:nix/hyprland/hyprland@0.53.3?flakeref=github:hyprwm/Hyprland"))
https://github.com/hyprwm/Hyprland/archive/dd220efe7b1e292415bd0ea7161f63df9c95bfd3.tar.gz

(Note that the flakeref qualifier only works for get_upstream_src_download_url, as "cache.nixos.org" only stores official packages, whereas flakeref usually points to third-party locations.)

For reference, the official Nix packages can be found at https://search.nixos.org/packages