move functions to nixpkgs

[Artturin]

now that NixOS/nixpkgs#167947 has been merged there is now a lib/flakes.nix

assigning myself

[jessestricker]

How does this relate to NixOS/nixpkgs#123117?

[srid]

It looks like this is blocked by needing to convince nixpkgs maintainers to allow flakes-specific functions in lib. See NixOS/nixpkgs#173006 (comment)