JLink: IllegalArgumentException thrown if modules with the same packages are encountered on the plugin module path

JLink: IllegalArgumentException thrown if modules with the same packages are encountered on the plugin module path

To reproduce:
1. create two modules which provides two services which are in the same package, but in different modules;
2. run jlink --plugins-modulepath . --list-plugins

java.lang.IllegalArgumentException: Modules customplugin_1 and customplugin both contain package plugin
        at java.lang.ModuleClassLoader.<init>(java.base@9.0/ModuleClassLoader.java:119)
        at java.lang.ModuleClassLoader.<init>(java.base@9.0/ModuleClassLoader.java:165)
        at java.lang.ModuleClassLoader.<init>(java.base@9.0/ModuleClassLoader.java:147)
        at jdk.tools.jlink.TaskHelper.createPluginsLayer(jdk.jlink@9.0/TaskHelper.java:815)
        at jdk.tools.jlink.TaskHelper$PluginsOptions.<init>(jdk.jlink@9.0/TaskHelper.java:173)
        at jdk.tools.jlink.TaskHelper$PluginsOptions.<init>(jdk.jlink@9.0/TaskHelper.java:152)
        at jdk.tools.jlink.TaskHelper$OptionsHelper.handleOptions(jdk.jlink@9.0/TaskHelper.java:501)
        at jdk.tools.jlink.JlinkTask.run(jdk.jlink@9.0/JlinkTask.java:186)
        at jdk.tools.jlink.Main.run(jdk.jlink@9.0/Main.java:48)