archetype:generate doesn't support inserting a module into a multi-module build like :create did

eg:
mvn archetype:create -DartifactId=parent -DgroupId=test
cd parent
mvn archetype:create -DartifactId=child

this would:

• add the new module
• update the pom with the parent POM reference (and inherit the groupId, version by default)
• update the parent POM to add the new <module> reference for the child.

:generate prompts for groupId/version