protected function createUnifiedNamespaceSubDirectory($unifiedSubNamespace)
    {
        $this->validateOutputDirectoryPermissions();

        $unifiedSubNamespacePath = $this->outputDirectory . str_replace('\\', DIRECTORY_SEPARATOR, $unifiedSubNamespace);
        $this->fileSystem->mkdir($unifiedSubNamespacePath, 0755);

        return $unifiedSubNamespacePath;
    }