While I'm not an enthusiast of Java modularization, unfortunately Java libraries that aren't module-aware are increasingly being a source of compiler warnings as people migrate to newer JDKs.

It would help if dom4j set up an automatic module name in the Jar manifest, like:

Automatic-Module-Name: org.dom4j

I'm not familiar with gradle, but apparently the following would work:

jar {
  manifest {
    attributes('Automatic-Module-Name': 'org.dom4j')
  }
}

See https://guides.gradle.org/building-java-9-modules/#optional_add_code_automatic_module_name_code_manifest_entries_for_all_other_projects

Of course that a full module-info would also be a good thing, but the above solution is simple and solves the main issue of having to rely on generated module names.