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
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