nixvim lib with no pkgs dependency

[MattSturgeon]

It is conventional for lib in most flake outputs to not be system-specific.

It is also problematic if pkgs is (unnecessarily) evaluated multiple times to build someones config.

Currently, to evaluate nixvim's lib we need both our flake outputs and pkgs. Since we then also use nixvim's lib to evaluate the modules, we need a pkgs outside of (before) the module eval.

We also need nixvim's lib in some parts of our flake eval (e.g. in some of our test derivations), so that means we need to evaluate a pkgs for our flake. Now that I put that in writing, it doesn't sound so bad; this would always be the case anyway since flake evals almost always end up needing to evaluate pkgs for something...

Regardless, I think the ideal is to make it so that our flake's lib output is not an attrsof <system> but instead a library attrset that does not depend on system/pkgs.

To this end, we'd have to refactor or replace the standalone wrapper with a new design that accepts system as an argument, similar to nixpkgs.lib.nixosSystem or home-manager's equivalent. We'd also have to move any functions in helpers that need pkgs into module options (such as the config.lib module option) OR refactor them to take pkgs as a function argument.

E.g. helpers.writeLua could look something like: writeLua = pkgs: name: text: pkgs.runCommand name { inherit text; } "echo -n "$text" > $out && stylua $out";
(receiving pkgs as its first argument)

This matches the pattern used in nixpkgs lib, e.g. lib.callPackageWith or lib.types.mkPackageOption both take pkgs as their first argument and can be curried if needed within the module eval (or within pkgs, i.e. pkgs.callPackage = lib.callPackageWith pkgs).