support arbitrary downloads

[KiaraGrouwstra]

Currently, npins add supports a number of difference source types:

$ npins add --help
Adds a new pin entry

Usage: npins add [OPTIONS] <COMMAND>

Commands:
  channel  Track a Nix channel
  github   Track a GitHub repository
  forgejo  Track a Forgejo repository
  gitlab   Track a GitLab repository
  git      Track a git repository
  pypi     Track a package on PyPi
  tarball  Track a tarball

One option potentially missing here seems non-tarball downloads, which seems useful for e.g. NixOS ISO downloads.
To be fair, this may differ from existing source types in the sense it would offer a file (like pkgs.fetchurl) rather than a directory.

[piegamesde]

Do we have builtin fetchers that can do that? I'd like to avoid adding pin types that require Nixpkgs to work

[KiaraGrouwstra]

i think we have:

• builtins.fetchurl
• builtins.fetchtree with type=file;

[kissgyorgy]

I think this feature would be useful. I personally would like to track every external dependency with npins, so I can see what do I download from where in a single source of truth instead of sprinkling downloads all over nixfiles.
npins would also help updating them with getting the hashes.

[piegamesde]

I am open for having that feature as long as it can be made to work with non-flakey builtin fetchers. (I've heard bad things about fetchtree, but I would consult other Lix core team members on this before completely rejecting it)