This option provides a more convenient way to overlay dummy packages.
It also adds a function `config.lib.test.mkStubPackage` that can,
e.g., be used for `package` options.
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.
