Validateable classes and command objects from plugins are not automatically scanned

If a plugin defines command objects or @Validateable classes, the application does not evaluate these as validateable by default and the result can be very confusing error messages about missing properties as the "constraints" property remains a Closure instead of a constrainedProperties map.

Example: plugin that has command objects and uses bean-fields. After installing this plugin the plugin's forms will not work and show obscure error about missing property.

Recommendation: either during plugin packaging time or plugin compile time, plugin artefacts are always scanned/info put into metadata such that the application author does not need to update their Config grails.validateable.packages to make plugins work.