Declare Spring modules with JDK 9 module metadata

JDK 9's Jigsaw initiative aims to allow for module metadata (module-info.java) that can be added to framework and library jars while still keeping them compatible with JDK 8. Let's do this for Spring Framework 5.0's modules as far as feasible. However, we might not be able to express our optional dependency arrangement that way, in which case we might have to resort to an "automatic modules" approach for the more modest purposes of SPR-13716.