You can implement your commandlet in your plug-in. You can also have your commandlet in a separate module, and then have a third module that contains functionality that is shared by both the plugin and the commandlet.
Remember, a plug-in directory can contain multiple modules. It is OK for one module in this folder to link to another module in that folder. The only thing that is not OK is for some module outside of your plug-in’s folder to link to a module inside the plug-in’s folder.
I hope that makes sense.