This removes the hard-coded fallback Home Manager paths. Specifically
${XDG_CONFIG_HOME:-$HOME/.config}/nixpkgs/home-manager
and
"$HOME/.nixpkgs/home-manager"
Use `@HOME_MANAGER_PATH@` if it has been provided and points to
something that exists. Warn the user if it does not point to
something.
If we have not been provided with a `@HOME_MANAGER_PATH@` that exists,
then for both hard-coded paths show a warning if something exists
where the paths are pointing.
This no longer attempts to use either of the hard-coded paths as
fallback for the Home Manager path.
1e7ba7102e