Auto-export will create def with duplicate root path if existing def has no properties

If an existing local def has no properties, it is not added to the back-references in ConfigurationNode.getDefinitions(). If a property is then added/updated on the node, DefinitionMergeService creates a new local def for that property instead of reusing the existing def. If the new def is in the same module as the existing one, this is a config error and auto-export fails with an exception on reload.

This can be trivially reproduced with a new archetype-generated project by adding a (dummy) property to 

/hippo:configuration/hippo:modules/autoexport

(the parent, not the hippo:moduleconfig child)