I'm trying to following this wiki page to configure home-manager via `configuration.nix`: https://nixos.wiki/wiki/Home_Manager

I believe I have it set up correctly, with the import and `home-manager.users.my_username = { ... }`, but, then this happens.

```
$ home-manager switch         
No configuration file found. Please create one at /home/my_username/.config/nixpkgs/home.nix
```

`home-manager.users` seems to be defined here, but I can't quite grok it: https://github.com/rycee/home-manager/blob/master/nixos/default.nix