We now consider all registered PersistentEntity instances and try to detect repository metadata for them. A case we didn't cover before was that a repository was declared for an aggregate super type but the actual child aggregate class didn't have a dedicated repository declared. While this is a perfectly valid scenario, the mapping information was broken as it fell back on the plain domain type information and produced paths and relation names derived from that, even if there's a super type repository available.
Thus, solely traversing the aggregate types we have repositories registered for is not enough. We now traverse all known PersistentEntity types and also register repository metadata for all types that are assignable to a known domain type. The latter is actually implemented in Spring Data Commons' Repositories via spring-projects/spring-data-commons#2406.
Nested entities that contain a reference to an aggregate root get a link to that attached to their representation. Previously, the creation of those links assumed that the reference is a materialized instance of the remote aggregate. That's now altered to be able to deal with associations, use identifiers directly or materialize to an intermediate aggregate instance to potentially use a custom lookup.
RepositoryRestConfiguration now defaults the LinkRelationProvider instance to a delegating one considering both AnnotationLinkRelationProvider and EvoLinkRelationProvider so that @Relation annotations on entity types.
Finally remove the dependency to Spring WebMVC from Spring Data REST's core module. Removed previously deprecated methods and update test configuration applying customizations.
Related tickets: DATAREST-1543.
Removed Java8PluginRegistry not in use anymore anyway. Same for RepositoryRestConfigurerAdapter. Deprecated legacy ResourceMappingUtils and its client code exposed in RepositoryRestConfiguration as it's also not considered anymore.
