Before this path would point to the modules path. Using the project
root instead makes it possible to set `<home-manager>` to point to a
downloadable archive of Home Manager. This should make it
significantly easier to install and keep Home Manager up to date.
To match this change we also deprecate the Home Manager option
programs.home-manager.modulesPath
and instead ask users to use
programs.home-manager.path
This command allows the user to examine the news items generated by
the news module. See #52.
Many thanks to @nonsequitur and @uvNikita for suggestions and
improvements.