code: public ContentPackageBuilder xmlNamespace(String prefix, String uri) {
      metadata.addXmlNamespace(prefix, uri);
      return this;
    }
docs: Register a XML namespace that is used by your content added to the JCR XML.
  This method can be called multiple times to register multiple namespaces.
  The JCR namespaces "jcr", "nt", "cq" and "sling" are registered by default.
  @param prefix Namespace prefix
  @param uri Namespace URI
  @return this