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.
* zsh: update prezto path structure
The path structure was changed in Nixpkgs and this commit updates
the module to match.
Fixes#1773
* zsh-prezto: fix tests, small tidyup
Co-authored-by: Nick Hu <me@nickhu.co.uk>