splice: defer per-output spliceReal recursion behind genAttrs (#508944)

            getOutputs (lib.optionalAttrs success value);

          getOutputs =

            value: lib.genAttrs (value.outputs or (lib.optional (value ? out) "out")) (output: value.${output});

          outputNames = defaultValue.outputs or (lib.optional (defaultValue ? out) "out");

          outputSplice = spliceReal (

            mapCrossIndex tryGetOutputs value' // { hostTarget = getOutputs value'.hostTarget; }

          );

        in

        # The derivation along with its outputs, which we recur

        # on to splice them together.

        if lib.isDerivation defaultValue then

          augmentedValue

          // spliceReal (mapCrossIndex tryGetOutputs value' // { hostTarget = getOutputs value'.hostTarget; })

          augmentedValue // lib.genAttrs outputNames (out: outputSplice.${out})

        else if lib.isAttrs defaultValue then

          spliceReal value'

        else