vimPluginsUpdater: make development easier

`nix develop .#vimPluginsUpdater` now lets you enter a shell where you
can run `pkgs/applications/editors/vim/plugins/update.py` and
iteratively develop !

- removed `warn` warning from python by using `warning` instead
- `plugin2nix` was calling the same bit of code over and over thus
  slowing down the generator by a lot

pkgs/development/lua-modules/updater/updater.py

@@ -177,7 +177,7 @@ def generate_pkg_nix(plug: LuaPlugin):
                 "'version' and 'ref' will be ignored as the rockspec is hardcoded for package %s"
                 % plug.name
             )
-            log.warn(msg)
+            log.warning(msg)
 
         log.debug("Updating from rockspec %s", plug.rockspec)
         cmd.append(plug.rockspec)