Core/ModuleExtensionCleanerDebug.php
protected function getModuleExtensionsGarbage($moduleMetaDataExtensions, $moduleInstalledExtensions)
    {

        $garbage = parent::getModuleExtensionsGarbage($moduleMetaDataExtensions, $moduleInstalledExtensions);

        foreach ($moduleInstalledExtensions as $coreClassName => $listOfExtensions) {
            foreach ($listOfExtensions as $extensions) {
                if (! (isset($moduleMetaDataExtensions[$coreClassName]) && $moduleMetaDataExtensions[$coreClassName] == $extensions)) {
                    $garbage[$coreClassName][] = $extensions;
                }
            }
        }

        return $garbage;
    }
protected function getModuleExtensionsGarbage($moduleMetaDataExtensions, $moduleInstalledExtensions)
    {

        $garbage = parent::getModuleExtensionsGarbage($moduleMetaDataExtensions, $moduleInstalledExtensions);

        foreach ($moduleInstalledExtensions as $coreClassName => $listOfExtensions) {
            foreach ($listOfExtensions as $extensions) {
                if (! (isset($moduleMetaDataExtensions[$coreClassName]) && $moduleMetaDataExtensions[$coreClassName] == $extensions)) {
                    $garbage[$coreClassName][] = $extensions;
                }
            }
        }

        return $garbage;
    }
Returns extension which is no longer in metadata - garbage

@param array $moduleMetaDataExtensions  extensions defined in metadata.
@param array $moduleInstalledExtensions extensions which are installed

@return array