PluginRepository负责加载nutch系统下的插件，可以通过installExtensionPoints（）函数查看要加载的插件和对应路径

  private void installExtensionPoints(List<PluginDescriptor> plugins) {    if (plugins == null) {      return;    }        for (PluginDescriptor plugin: plugins) {      for(ExtensionPoint point:plugin.getExtenstionPoints()) {        String xpId = point.getId();        LOG.debug("Adding extension point " + xpId);//        System.err.println("xPId="+xpId+"   |||   point="+plugin.getPluginPath());        fExtensionPoints.put(xpId, point);      }    }  }