This simplifies installing by (re-)enabling the standard practice
of using the main expression as a function returning the package derivation.
Convert the src dir path to a string to avoid needlessly copying the whole
dir to the nix store.
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
