* systemd: add `packages` option
The `systemd.user.packages` option is the Home Manager equivalent of
NixOS’s `systemd.packages` option and provides a way to specify packages
providing systemd user units.
This option is similar to `dbus.packages`.
* systemd: only create the parent directory if there are packages to symlink
Due to the defaults in `systemd.user.settings`, the default value when
there are no settings explicitly set is `{ Manager = { }; }`. This
means an empty file is created even when `systemd.user.settings` is
never used in home-manager configuration. Since user’s `user.conf` is
preferred to the global `/etc/systemd/user.conf`, this can cause any
values set in the latter to be discarded.
The `systemd.user.extraConfig` provides a way to generate a
`systemd-user.conf(5)` file for the user.
This is the home-manager equivalent of NixOS’s option of the same
name, with the difference that NixOS’s option generates a `user.conf`
file that is shared between all users.
