findUniqueIdentifier(packageXml) {
    var uniqueIdentifierId = packageXml.documentElement.getAttribute("unique-identifier");

    if (!uniqueIdentifierId) {
      return "";
    }

    var identifier = packageXml.getElementById(uniqueIdentifierId);

    if (!identifier) {
      return "";
    }

    if (identifier.localName === "identifier" && identifier.namespaceURI === "http://purl.org/dc/elements/1.1/") {
      return identifier.childNodes.length > 0 ? identifier.childNodes[0].nodeValue.trim() : "";
    }

    return "";
  }
Find Unique Identifier
@private
@param  {node} packageXml
@return {string} Unique Identifier text