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.
