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.
(cherry picked from commit 221056c59f)
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.
