Write file and symlinks from expressions

[Misterio77]

Building upon https://github.com/Infinidoge/nix-minecraft/pull/18.

Diff from #18 to this: https://github.com/Infinidoge/nix-minecraft/compare/Misterio77:deref-symlinks...Misterio77:file-formats

So, basically file/symlink items can now either be:

• The usual string-like (string, path, derivation) with the same behaviour we have today.
• A submodule that contains a value (that can be any sort of list/attrset) and a format (such as pkgs.formats.json { }).
  • The format may be null, in this case it will be infered (if possible) from the filename.

This is really really convenient for writing configuration files, as you can simply do them in nixlang without having to call any sort of function to write it.

Here's a real-world example:

{ pkgs, ... }: {
  services.minecraft-servers.servers.velocity = {
    enable = true;
    # ...
    files = {
      "velocity.toml".value = {
        config-version = "2.5";
        bind = "0.0.0.0:25565";
        player-info-forwarding-mode = "modern";
        forwarding-secret-file = "";
        forwarding-secret = "@VELOCITY_FORWARDING_SECRET@";
        online-mode = true;
        servers = {
          survival = "localhost:25561";
          limbo = "localhost:25560";
          try = [ "survival" "limbo" ];
        };
        forced-hosts = { };
        query = {
          enabled = true;
          port = 25565;
        };
        # ...
      };
      "plugins/librelogin/config.conf".value = {
        allowed-commands-while-unauthorized = [
          "login"
          "register"
          "2fa"
          "2faconfirm"
        ];
        auto-register = true;
        # ...
      };
      # We don't know what format .conf is, so specify it.
      "plugins/librelogin/config.conf".format = pkgs.formats.json { };
    };
  };
}

A nice consequence is that whitelist.json, server.properties, and eula.txt also don't have to be manually written with writeText.

TODO: document this in files and symlinks descriptions.

[Misterio77]

This should help us with adding comments: https://github.com/NixOS/nixpkgs/pull/224456

[Misterio77]

Hey @Infinidoge, I think it'll still take a while to merge the comments PR, do you think we can perhaps merge this one as-is? I can work on the comments later.

[Infinidoge]

Once again sorry for this taking so long! Thank you very much for the contribution, and for your patience with me!