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.
Setting `outputSpecified` prevents `getOutput` from descending into
outputs, which don't have an overridden `outPath`.
Additionally, use `__spliced` to permit derivations to use the dummy as
an input.
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.
