Ended up using this, but it would be nice if there was a clean way to extend lib in module args:

      _module.args = {
        inherit (self.lib) mkSystem mkLocalStagingSystem mkLocalSystem;
      };

Where are you supposed to add helper functions similar to mkShell? I noticed that nixpkgs puts it in pkgs/build-support/mkshell, rather than lib.
But if I do that, it will break commands like nix flake show:

 nix flake show
error: expected a derivation

I cannot put it under legacyPackages either, since Flakelight actually type checks it, and expects it to result in a derivation:

      error: A definition for option `legacyPackages' is not of type `nullable function that evaluates to a(n) Nixpkgs package set'. Definition values:
       - In `/nix/store/f199yb2qch3qa0wdq9cglg4wnr7cshfw-source/builtinModules/nixDir.nix':
           {
             mkPackageShell = <function, args: {inputs, lib, pkgs}>;
           }

Where are you supposed to add helper functions similar to mkShell?
I noticed that nixpkgs puts it in pkgs/build-support/mkshell, rather than lib.
But if I do that, it will break commands like nix flake show:

 nix flake show
error: expected a derivation

I cannot put it under legacyPackages either, since Flakelight actually type checks it, and expects it to result in a derivation:

      error: A definition for option `legacyPackages' is not of type `nullable function that evaluates to a(n) Nixpkgs package set'. Definition values:
       - In `/nix/store/f199yb2qch3qa0wdq9cglg4wnr7cshfw-source/builtinModules/nixDir.nix':
           {
             mkPackageShell = <function, args: {inputs, lib, pkgs}>;
           }

Where are you supposed to add helper functions similar to mkShell?
I noticed that nixpkgs puts it in pkgs/build-support/mkshell, rather than lib.
But if I do that, it will break commands like nix flake show:

 nix flake show
error: expected a derivation

I cannot put it under legacyPackages either, since Flakelight actually type checks it, and expects it to result package rather than a function:

      error: A definition for option `legacyPackages' is not of type `nullable function that evaluates to a(n) Nixpkgs package set'. Definition values:
       - In `/nix/store/f199yb2qch3qa0wdq9cglg4wnr7cshfw-source/builtinModules/nixDir.nix':
           {
             mkPackageShell = <function, args: {inputs, lib, pkgs}>;
           }

Where are you supposed to add helper functions similar to mkShell?
I noticed that nixpkgs puts it in pkgs/build-support/mkshell, rather than lib.
But if I do that, it will break commands like nix flake show:

 nix flake show
error: expected a derivation

I cannot put it under legacyPackages either, since Flakelight actually type checks it, and expects it to result in a package rather than a function:

      error: A definition for option `legacyPackages' is not of type `nullable function that evaluates to a(n) Nixpkgs package set'. Definition values:
       - In `/nix/store/f199yb2qch3qa0wdq9cglg4wnr7cshfw-source/builtinModules/nixDir.nix':
           {
             mkPackageShell = <function, args: {inputs, lib, pkgs}>;
           }

In reply to @niclas:overby.me
Did you come to a conclusion for this?

Some folk put per-system things in lib like lib.x86_64-linux.mkShell. You can do that in flakelight either with nix/lib.nix:

{ genSystems, ... }: ({
  # regular lib stuff
} // genSystems (pkgs: {
  # per-system lib stuff
}))