docs: def package_url(self):
          """Return the package URL associated with this metadata"""
  
          if self.resource_file == DEFAULT_METATAB_FILE or self.target_format in ('txt','ipynb'):
              u = self.inner.clone().clear_fragment()
              u.path = dirname(self.path) + '/'
              u.scheme_extension = 'metapack'
          else:
              u = self
  
          return MetapackPackageUrl(str(u.clear_fragment()), downloader=self._downloader)