code: public static String partialDeltemplateTemplateName(
        String delTemplateName, @Nullable String delPackageName, String variant) {
      String delPackageTemplateAndVariantStr =
          (delPackageName == null ? "" : delPackageName)
              + "_"
              + delTemplateName.replace('.', '_')
              + "_"
              + variant;
      delPackageTemplateAndVariantStr = delPackageTemplateAndVariantStr.replace('.', '_');
      // Generate the actual internal-use template name.
      return ".__deltemplate_" + delPackageTemplateAndVariantStr;
    }
docs: Returns the inferred 'partial' name for a deltemplate.