Provide better handling of incompatible visibilities between domain type and repository

[rweisleder]

Related to #2526

Given a package-private entity

@Entity
class Book {

    @Id
    private String isbn;

    // no-args-constructor, getter, setter etc.
}

and a public repository with a custom findBy method

public interface BookRepository extends CrudRepository<Book, String> {

    Book findByIsbn(String isbn);
}

then the invocation of the findBy method fails with an error message that is more cryptic than helpful:

java.lang.IllegalAccessError: failed to access class org.example.Book from class jdk.proxy2.$Proxy105 (org.example.Book is in unnamed module of loader 'app'; jdk.proxy2.$Proxy105 is in module jdk.proxy2 of loader 'app')

The cause comes from the different visibilities. If the entity is public or the repository is package-private, it works.

I would expect this construct to work out of the box, because if the repository is package-private, it works again. Or provide a helpful error message at startup indicating the incompatible visibilities.