code: public DocLink getExternalLink(String pkgName, DocPath relativepath, String filename) {
          return getExternalLink(pkgName, relativepath, filename, null);
      }
docs: Convert a link to be an external link if appropriate.
  
  @param pkgName The package name.
  @param relativepath    The relative path.
  @param filename    The link to convert.
  @return if external return converted link else return null