review cases where Java/Scala packages are split across 2 or more jars

Java Modules (JPMs) discourage having packages where classes appear in different jars. It is a security risk to have the possibility of rogue jar including classes that hide classes in a well known jar.

cluster-typed and distributed-data share the package
* org.apache.pekko.cluster.ddata
* we may be ok as is because the cluster-typed packages are all grouped under `org.apache.pekko.cluster.ddata.typed` and there are packages in distributed-data that appear under `org.apache.pekko.cluster.ddata.typed` or packages that are grouped under it

There may are more instances. This is just one that I spotted today.

Related:
* https://github.com/apache/pekko-http/issues/733

Start by comparing the package layouts of the cluster-typed and distributed-data modules, especially org.apache.pekko.cluster.ddata and its typed subpackages. Search the project for other Java/Scala packages present in multiple jars, then assess each case for the Java Modules security concern. Done means the split-package cases are identified and reviewed.

Written by the indexing model from the issue text.