stdenv.mkDerivation: simplify overrideAttrs with extends and toOverlay

      #              ^^^^

      overrideAttrs = f0:

        let

          f = self: super:

            # Convert f0 to an overlay. Legacy is:

            #   overrideAttrs (super: {})

            # We want to introduce self. We follow the convention of overlays:

            #   overrideAttrs (self: super: {})

            # Which means the first parameter can be either self or super.

            # This is surprising, but far better than the confusion that would

            # arise from flipping an overlay's parameters in some cases.

            let x = f0 super;

            in

              if builtins.isFunction x

              then

                # Can't reuse `x`, because `self` comes first.

                # Looks inefficient, but `f0 super` was a cheap thunk.

                f0 self super

              else x;

        in

        makeDerivationExtensible

            (self: let super = rattrs self; in super // (if builtins.isFunction f0 || f0?__functor then f self super else f0));

          (lib.extends (lib.toExtension f0) rattrs);

      finalPackage =

        mkDerivationSimple overrideAttrs args;