nix-bitcoin.nix: add defaultText (automatic)

This enables generating module option documentation.

This commit was genereated by running the following script inside the
repo root dir:

def add_default_text(file)
  src = File.read(file)
  src2 = src.gsub(/( = mkOption\s+\{[^{]*?)(\n\s+default = )(.*?);$(.*?\})/m) do |str|
    pre, defaultVar, default, post = Regexp.last_match.captures
    replacement =
      if !post.include?('defaultText =')
        if default =~ /\bpkgs\b/
          defaultText = default.lines.length == 1 ? default : "(See source)"
          "#{pre}#{defaultVar}#{default};#{defaultVar.sub('default', 'defaultText')}#{defaultText.inspect};#{post}"
        end
      end
    replacement or str
  end
  File.write(file, src2) if src2 != src
end

Dir["modules/**/*.nix"].each do |f|
  next if File.basename(f) == "nix-bitcoin.nix"
  add_default_text f
end

modules/lightning-pool.nix

@@ -27,6 +27,7 @@ let
     package = mkOption {
       type = types.package;
       default = config.nix-bitcoin.pkgs.lightning-pool;
+      defaultText = "config.nix-bitcoin.pkgs.lightning-pool";
       description = "The package providing lightning-pool binaries.";
     };
     dataDir = mkOption {
@@ -54,6 +55,7 @@ let
           --network ${network} \
           --basedir '${cfg.dataDir}' "$@"
       '';
+      defaultText = "(See source)";
       description = "Binary to connect with the lightning-pool instance.";
     };
     tor = nbLib.tor;