Only depend on nixpkgs/lib.

This reflects the actual requirements of the core flake-parts
functionality and removes any possible confusion about where lib comes
from (always flake-parts's input) vs where pkgs comes from (always the
flake's nixpkgs input). Flakes which use flake-parts should only ever
have to override its input in the very unlikely event of a bug in lib.

Hopefully lib will be separated into its own flake some day, which
will also make this a much smaller footprint.

template/multi-module/flake.nix

@@ -2,7 +2,6 @@
   description = "Description for the project";
 
   inputs = {
-    flake-parts.inputs.nixpkgs.follows = "nixpkgs";
     nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
   };