Instead of having to manually stub packages that should not be
downloaded we instead automatically stub all packages (except a small
list of whitelisted ones). Tests can re-introduce the real package by
using the `realPkgs` module argument.
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.
