code: public String getPackageFileHeadName(PackageDoc packageDoc) {
          return packageDoc == null || packageDoc.name().length() == 0 ?
              DocletConstants.DEFAULT_PACKAGE_FILE_NAME : packageDoc.name();
      }
docs: Given a package, return its file name without the extension.
  @param packageDoc the package to check.
  @return the file name of the given package.