Can't make it work with flake parts

[pavukach]

I try to redo setup using flake-parts, as wiki does not mention how to enable lock flattening with default flakes and in general favors this way of installation. i have something like that:

inputs:
let
  system = "x86_64-linux";
  username = "maksym";
  nixDir = "/home/${username}/nixos";
  pkgsConfig = {
    allowUnfree = true;
  };
in
{
  imports = [ ./inputs.nix ];
  systems = [ system ];
  flake = {
    nixosConfigurations.nixos = inputs.nixpkgs.lib.nixosSystem {
      inherit system;
      specialArgs = {
        inherit
          inputs
          username
          pkgsConfig
          nixDir
          ;
        inherit (inputs) self;
      };
      modules = [
        (inputs.import-tree ./host)
        (inputs.import-tree ./modules)
        (inputs.import-tree ./secrets)
      ];
    };
  };
}

And, when i try to run this i get error: attribute 'nixpkgs' missing for nixosConfigurations.nixos = inputs.nixpkgs.lib.nixosSystem.

part of inputs.nix:

{

  flake-file.inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
  };
}

Maybe I am not smart enough, but in general documentation omits too much as an exercise to the reader. For example, no templates actually show usage of inputs you added