rework nixd development tooling for dendritic pattern

[hyperparabolic]

Manual configuration of nixd that includes auto-completion for modules can only specify one host for discovering options, and can only discover the modules that are actually imported on that specific host. Traditional patterns learn toward modules always being imported, and just enabling / disabling behavior with options, so this hasn't usually been an issue. However, optionally importing modules seems pretty core to convenient dendritic design patterns.

This isn't an issue yet, but eventually if I make options for modules that aren't imported on oak, nixd won't support option completion for those modules.

I think this can be solved by creating a dummy host that imports every module, and referencing that host as a part of your nixd config. This might get a little sticky if there are ever conflicting options in different modules, but lib.mkOverride should be able to take care of this, and I think it's a price worth paying for lsp support.

Mulling over this for a second to think about other alternatives that won't increase nix flake check evaluation time.