dubbo SPI配置多个tag无法正确解析

[zishinan]

• I have searched the issues of this repository and believe that this is not a duplicate.
• I have checked the FAQ of this repository and believe that this is not a duplicate.

Environment

• Dubbo version: master分支
• Operating System version: windows
• Java version: JDK1.8

Steps to reproduce this issue

1. 在spi service配置中配置
   impl,implMultiName=org.apache.dubbo.common.extension.ext10.impl.Ext10MultiNamesImpl
2. ExtensionLoaderTest中加测试用例
   Ext10MultiNames ext10MultiNames = ExtensionLoader.getExtensionLoader(Ext10MultiNames.class).getExtension("impl");
   Assertions.assertNotNull(ext10MultiNames);
   3.运行不通过

Expected Result

应该获取到对应的实现

Actual Result

获取到空，测试不通过

[zishinan]

fixed