modules/lsp/hls: port ghcPackage option from plugins.lsp

[MattSturgeon]

WIP PR to begin porting plugins.lsp server-specific options to the new lsp module. Depends on #3783.

Open questions:

• Do we still want installXyz and xyzPackage options, or just nullable package options?
• Which options should be ported to lsp and which to plugins.lspconfig?

[sportshead]

IIRC from my previous testing installing hls will cause a version (not sure if its the default nixpkgs one) of ghc to be in the closure, since hls has a dependency on ghc. Therefore I don't think installGhc actually does anything apart from possibly wasting another ~3gb of disk space, since the extra ghc will only be available in nvim (where it isn't needed) anyway.

[sportshead]

Had a quick test and hls works with installGhc = false (using plugins.lsp). I do think packageFallback is much more useful for haskell – hls loves to complain whenever the bundled ghc version is not the exact same as the workspace ghc version.

[saygo-png]

hls works with installGhc = false

I've tested it and it does not. I used nix-shell with --pure to get a clean environment. You most likely have ghc in your path from somewhere else, which is why it worked for you.

That said I much prefer a nullable package here instead of an install option. I think it's more idiomatic and consistent with the rest of Nixvim and other Nix projects like home-manager.