Add support for completing nixosModules.default.options

[kjeremy]

Is your feature request related to a problem? Please describe.

I would like completion for a module that I am exposing from a flake.

Describe the solution you'd like

This should expose my options but it doesn't seem to (or possibly this should just work).

"nix.serverSettings": {
        "nixd": {
            "options": {
                "my-cool-module": {
                    "expr": "(builtins.getFlake \"/home/jkolb/ara/ipd-nix\").nixosModules.default.options",
                }
            }
        }
    }

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.

[inclyc]

I would like completion for a module that I am exposing from a flake.

Please show nix-repl for that module.

(builtins.getFlake \"/home/jkolb/ara/ipd-nix\").nixosModules.default.options

For example,

nix-repl> (builtins.getFlake "/home/jkolb/ara/ipd-nix").nixosModules.default.options

What is the result?

[blindFS]

@inclyc I have similar issues with nix-darwin.
I'd like to have completion for its options as well as home-manager's darinmodule options
Not sure I'm using the correct way for it, but I can get only completions for package names working.
Here's my Neovim LSP config:

      lspconfig.nixd.setup({
        cmd = { "nixd" },
        filetypes = { "nix" },
        settings = {
          nixd = {
            formatting = {
              command = { "nixfmt" },
            },
            options = {
              darwin = {
                expr = string.format(
                  '(builtins.getFlake "%s/nix").darwinConfigurations.%s.options',
                  vim.env.HOME,
                  vim.loop.os_gethostname()
                ),
              },
            },
          },
        },
      })

The command in nix-repl generates following output:

[0.0 MiB DL]{
  _module = { ... };
  assertions = { ... };
  borders = { ... };
  documentation = { ... };
  environment = { ... };
  fonts = { ... };
  home-manager = { ... };
  homebrew = { ... };
  ids = { ... };
  launchd = { ... };
  lib = { ... };
  meta = { ... };
  networking = { ... };
  nix = { ... };
  nix-homebrew = { ... };
  nixpkgs = { ... };
  power = { ... };
  programs = { ... };
  security = { ... };
  services = { ... };
  system = { ... };
  time = { ... };
  users = { ... };
  warnings = { ... };
}

[blindFS]

Update: just found out that it works for me at top-level, but not in a module's config = {}