CMSModuleLoader is quite old on its own and depends which class is custom module trying to extend... If you are trying to run the site, rebuild should be enough, though it depends on context of code. For example "Registering event handlers using the CMSModuleLoader class only works in the App_Code folder of the Kentico web project (CMS or CMSApp)." To make project more future proof or upgradable, please consider running customizations as custom module as David mentions in his post.