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);
}
}
}