Add option to enforce explicit module dependency declarations

Discussed in https://github.com/spring-projects/spring-modulith/discussions/923

^{Originally posted by ghferrari November 4th, 2024}
First of all, thanks to the Spring Modulith team for this great package. :-)

If I've understood the documentation for version 1.2.5, Spring Modulith currently uses an “allow-by-default” access model i.e. access to other modules is allowed unless explicitly restricted. Is there a way to switch to a "deny-by-default” permission model? In this scenario, access to other modules would be forbidden by default unless specifically granted? I think this would be a useful feature.