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/btcpayserver.nix

@@ -21,6 +21,7 @@ let
                     config.nix-bitcoin.pkgs.btcpayserver.override { altcoinSupport = true; }
                   else
                     config.nix-bitcoin.pkgs.btcpayserver;
+        defaultText = "(See source)";
         description = "The package providing btcpayserver binaries.";
       };
       dataDir = mkOption {
@@ -69,6 +70,7 @@ let
       package = mkOption {
         type = types.package;
         default = config.nix-bitcoin.pkgs.nbxplorer;
+        defaultText = "config.nix-bitcoin.pkgs.nbxplorer";
         description = "The package providing nbxplorer binaries.";
       };
       address = mkOption {