[lihaoyi]

You can, but why should you need to? Why can't the build tool take the plugin code directly off of disk, build it, and use it? This kind of orchestration of manual steps is what build tools are meant to be good at

[cyberax]

Sure. But adding ability to self-modify the build drastically increases the complexity of a build tool. Maven developers decided that they want to avoid that.

[beeboobaa3]

It can, for plugins. GP is talking about extensions which you typically don't need.