When setting `...sway.package = null`, the default bar configuration
would throw an error trying to use the bar from the null package.
Logic is added to use the bar from `pkgs.sway` instead of `cfg.package`
if it is null.
Fixes#1714
Using the final package in the `onChange` block broke some use cases.
This restores the old behavior and instead solves the test
dependencies in a different way.
Fixes#1611
This reverts commit 7c3c64208e.
