Interfaces must only be defined in implementation packages if:
there are multiple implementations (to define a common contract)
or they make testing (mocking) much easier
or they are defined in an other package than one of the implementations (to break dependency cycles).
Interfaces that do not fit in one of the above categories should be removed. Some examples come to mind:
org.hibernate.search.backend.lucene.search.query.impl.LuceneSearchResultExtractor (on contrary to its Elasticsearch counterpart, it's defined in the same package as its implementation)
Maybe let's add a JQAssistant rule to detect that, too?