Update to the new NUR format

See https://github.com/nix-community/NUR/issues/45

default.nix

@@ -1,19 +1,21 @@
 # This file describes your repository contents.
-# It should return a set of nix derivations.
-# It should NOT import <nixpkgs>. Instead, you should take all dependencies as
-# arguments.
+# It should return a set of nix derivations
+# and optionally the special attributes `lib`, `modules` and `overlays`.
+# It should NOT import <nixpkgs>. Instead, you should take pkgs as an argument.
+# Having pkgs default to <nixpkgs> is fine though, and it lets you use short
+# commands such as:
+#     nix-build -A mypackage
 
-{ callPackage
-, libsForQt5
-, haskellPackages
-, pythonPackages
-# , ...
-# Add here other callPackage/callApplication/... providers as the need arises
-, ... }:
+{ pkgs ? import <nixpkgs> {} }:
 
 {
-  example-package = callPackage ./pkgs/example-package { };
-  # some-qt5-package = libsForQt5.callPackage ./pkgs/some-qt5-package { };
+  # The `lib`, `modules`, and `overlay` names are special
+  lib = import ./lib { inherit pkgs; }; # functions
+  modules = import ./modules; # NixOS modules
+  overlays = import ./overlays; # nixpkgs overlays
+
+  example-package = pkgs.callPackage ./pkgs/example-package { };
+  # some-qt5-package = pkgs.libsForQt5.callPackage ./pkgs/some-qt5-package { };
   # ...
 }