lib: aliased options don't provide their options via getSubOptions

[llakala]

Context

type.getSubOptions is a reliable way to access the options that a submodule provides. Here's an example of its usage within a repl environment:

nix-repl> nixosConfigurations.HOSTNAME.type.getSubOptions []
{
  _module = { ... };
  appstream = { ... };
  assertions = { ... };
  baseVars = { ... };
  boot = { ... };
  console = { ... };
  containers = { ... };
# etc, more nixos module options

nixd relies on this function for providing completions for submodules.

Example

lib provides a function mkAliasOptionModule. I personally use this within my configuration to give quick access to home-manager settings (aliasing home-manager.users.MYUSERNAME to hm. However, the buggy behavior comes from any alias, so I'll use the example of aliasing environment to env.

This alias could be created like this:

imports =
[
  ( lib.mkAliasOptionModule ["env"] ["environment"] )
];

This works, and now this would be a valid use of the option:

env.systemPackages = with pkgs; 
[
  git
]; 

The issue

When inspecting this alias via the repl, getSubOptions does not work as expected.:

nix-repl> nixosConfigurations.HOSTNAME.options.env.type.getSubOptions []
{
  _module = { ... };
}

The Nixd completions should show something like this:

But instead, they show this:

I don't believe this is a fault of Nixd, but instead a fault of mkAliasOptionModule. getSubOptions provides a unified experience in every other case, so it should be expected that it works the same for an aliased option.

Additional context

See nix-community/nixd#615 where I first brought this issue up, and was recommended to create an issue here.

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