Grails 3 importFrom in Command Object constraints

I used to be able keep command objects separate from controllers in src/groovy in Grails 2.5.x. Now when trying to import constraints within command objects placed in src/main/groovy I get the error groovy.lang.MissingMethodException.

My command object is as follows:

class ProfileCommand implements Validateable {

    String lastName
    String firstName

    static constraints = {
        importFrom Profile
    }
}

I did see the following line in the validation guide for grails 3.1.6

It is important that the application register the validateable classes with the framework. Simply defining the constraints property is not sufficient.

Where do I go about registering these classes then?