Harmonized maven dependency management

This commit harmonizes the dependency management of internal modules so that versions can be omitted everywhere. 

Update the maven coordinates to provide the full groupId for consistency

This also means that `spring-boot-dependencies` can be used directly as a source for any module that boot provides, which is a good thing. The starter pom now only contains the further customizations that we want to push as "good" defaults. Previously, the two were provided by the same infrastructure.