Validation fails for reusable recipes

[vincent-zurczak]

It says root components must have the target installer.
Which is right. But reusable recipes are incomplete applications.

[vincent-zurczak]

The problem is that the plug-ins cannot detect whether the project is a reusable recipe or not. The most precise option would be to check the POM. But not all Roboconf projects need to be Maven projects. Only reusable recipes have to. 🤒

And reading the POM would make us depend on m2e. 😒
An acceptable solution would be to verify the existence of a descriptor and instances. If none of them exist, we can assume it is a reusable recipe.