ensure that packages are not split across jars #733
Description
We have at least some packages split across pekko-http and pekko-http-core, eg
- org.apache.pekko.http.javadsl.settings
- org.apache.pekko.http.scaladsl.settings
With Java modules (JPMs), the idea is that packages are implemented in just one jar. This is a useful security feature because it stops rogue lib maintainers from sneakily adding variants of classes from other libs in their libs.