Don't try to validate a limited set of hardcoded options, instead just
convert them as-is. Now, users can keep all their options in a single
attribute set, including arbitrary `user_{option}`s which was impossible
to express with a hard-coded submodule. As a plus, there is also less
maintainence burden.
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.