原因是mybatis-generator-maven-plugin被放在了<pluginManagement/>里面，应该直接放在<plugin/>里面
mybatis-generator-maven-plugin
<pluginManagement/>
<plugin/>
参考：
https://stackoverflow.com/questions/50146453/why-intellij-idea-cannot-generate-a-maven-plugin
https://stackoverflow.com/questions/10483180/maven-what-is-pluginmanagement
