code: def fetch_build_egg(self, req):
          
          from setuptools.command.easy_install import easy_install
          dist = self.__class__({'script_args': ['easy_install']})
          opts = dist.get_option_dict('easy_install')
          opts.clear()
          opts.update(
              (k, v)
              for k, v in self.get_option_dict('easy_install').items()
              if k in (
                  # don't use any other settings
                  'find_links', 'site_dirs', 'index_url',
                  'optimize', 'site_dirs', 'allow_hosts',
              ))
          if self.dependency_links:
              links = self.dependency_links[:]
              if 'find_links' in opts:
                  links = opts['find_links'][1] + links
              opts['find_links'] = ('setup', links)
          install_dir = self.get_egg_cache_dir()
          cmd = easy_install(
              dist, args=["x"], install_dir=install_dir,
              exclude_scripts=True,
              always_copy=False, build_directory=None, editable=False,
              upgrade=False, multi_version=True, no_report=True, user=False
          )
          cmd.ensure_finalized()
          return cmd.easy_install(req)
docs: Fetch an egg needed for building