cataclysmDDA: add very basic framework for packaging mods

Add new namespace 'cataclysmDDA', in which package builders, games, and
mods are listed.

pkgs/games/cataclysm-dda/wrapper.nix

@@ -0,0 +1,32 @@
+{ lib, symlinkJoin, makeWrapper }:
+
+unwrapped:
+
+pkgsSpec:
+
+let
+  mods = if lib.isFunction pkgsSpec
+         then pkgsSpec unwrapped.pkgs
+         else pkgsSpec;
+in
+
+if builtins.length mods == 0
+then unwrapped
+else symlinkJoin {
+  name = unwrapped.name + "-with-mods";
+
+  paths = [ unwrapped ] ++ mods;
+
+  nativeBuildInputs = [ makeWrapper ];
+
+  postBuild = ''
+    if [ -x $out/bin/cataclysm ]; then
+        wrapProgram $out/bin/cataclysm \
+            --add-flags "--datadir $out/share/cataclysm-dda/"
+    fi
+    if [ -x $out/bin/cataclysm-tiles ]; then
+        wrapProgram $out/bin/cataclysm-tiles \
+            --add-flags "--datadir $out/share/cataclysm-dda/"
+    fi
+  '';
+}