We cannot guarantee that the Nix store will be writable during startup
so installing the user packages through `nix-env -i` may fail.
Therefore, when building through the NixOS module install through the
`users.users.<name?>.packages` option.
This removes the `nixosSubmodule` option in favor of a new option
`submoduleSupport.enable`. This name better indicates that the
submodule mode applies to both NixOS and nix-darwin.
This is a NixOS module that is intended to be imported into a NixOS
system configuration. It allows the system users to be set up directly
from the system configuration.
The actual profile switch is performed by a oneshot systemd unit per
configured user that acts much like the regular `home-manager switch`
command.
With this implementation, the NixOS module does not work properly with
the `nixos-rebuild build-vm` command. This can be solved by using the
`users.users.<name?>.packages` option to install packages but this
does not work flawlessly with certain Nixpkgs packages. In particular,
for programs using the Qt libraries.
