The ant dependency is only used for the excelant tasks  (which are
never used in Debian ). Any use of these tasks implicitly means Ant
is already on the classpath. In this context Ant can be seen as a
runtime, much like the Servlet API for web based stuff. So I think it
makes sense to remove the dependency from the binary package.
Splitting ant into ant+libant-java is a good idea, but this can probably
wait for the Buster development cycle.
This is the maintainer address of Debian's Java team
debian-j...@lists.debian.org for discussions and questions.