update-luarocks-packages: use pluginupdate.py

Trying to reuse the update scripts used by kakoune/vim to provide the
user with an unified convergence. Some stuff doesn't work yet (parallel
download, caching) but I (anyone else welcome to try too) will improve
it in other PRs.

pkgs/development/lua-modules/overrides.nix

@@ -347,4 +347,8 @@ with super;
       sourceRoot=./readline-2.6
     '';
   });
+
+
+  # aliases
+  cjson = super.lua-cjson;
 }