[regression] Plugins are not executed in POM order

<plugin>
  <groupId>org.codehaus.groovy.maven</groupId>
  <artifactId>gmaven-plugin</artifactId>
  <executions>
    <execution>
      <goals>
        <goal>generateStubs</goal>
        <goal>compile</goal>
        <goal>generateTestStubs</goal>
        <goal>testCompile</goal>
      </goals>
    </execution>
  </executions>
</plugin>
<plugin>
  <groupId>org.apache.maven.plugins</groupId>
  <artifactId>maven-plugin-plugin</artifactId>
  <executions>
    <execution>
      <goals>
        <goal>descriptor</goal>
        <goal>helpmojo</goal>
      </goals>
    </execution>
  </executions>
</plugin>