feat: allow nixpkgs package definitions

[OsiPog]

I found it quite annoying recently that it's not possible to copy a Nix package from nixpkgs directly into the packages/ directory because Blueprint only accepts the custom argument attributes.

Nixpkgs defines packages in a way that the function argument attrset is direct subset of nixpkgs which Blueprint currently doesn't support. I found the fix for this quite easy.

To test this you can add hello.nix to your packages/. And run git add packages/hello.nix && nix build .#hello

[OsiPog]

@zimbatm Can you review this, please? I think this is a great addition for compatibility with nixpkgs

[yajo]

FWIW this will still not be helpful if you're copying a pythonPackage definition, where the scope in nixpkgs is wider than just normal packages. Just an example; the same is true for other package sets in nixpkgs.

Personally I just use defaults:

{pkgs, lib ? pkgs.lib, hello ? pkgs.hello, ...}: lib.traceVal hello

Still I agree something could be improved.

[zimbatm]

What happens if you define a local hello package, should it override the hello attribute coming from nixpkgs? It could also be confusing to know what is coming from where on larger code bases.

[OsiPog]

As I see it when one wants to use the local hello package, then perSystem.packages.hello or flake.packages.${system}.hello needs to be used. All plain attrset inputs come from nixpkgs directly.