Don't expose a plugins(Closure) method in the build script formally.

 Exposing this formally (i.e. via Script, or script target) means that “plugins” can't be used as an identifier in a domain object container as our dispatch logic for creation in the container DSL will prefer this which is not what we want.

Instead, make the plugin(Closure) method implicitly available by attaching it to the distinct script class that is used when executing this block (plus the buildscript {} block).