use non-clashing core jar name across mapmate, messagemate, httpmate

[lestephane]

In the absence of module-info.java and Automatic-Module-Name in the MANIFEST.MF,
the jar name part before the version string will determine what module name
a jar is imported as in a modularized java application.

mapmate core, httpmate core, message core all use core-x.y.z.jar as jar filename.
So they can't be loaded in a modularized java application because their automatically
derived module name clashes.

Automatic-Module-Name Derivation is not handled by the javadoc command correctly (as Richard reported):

Exit code: 1 - error: cannot determine module name for /home/richard/projects/envimate/opensource/message-mate/core/target/core-1.0.9.jar
[ERROR] 
[ERROR] Command line was: /home/richard/apps/java/installed_versions/jdk-11.0.4+11/bin/javadoc @options @packages
[ERROR] 
[ERROR] Refer to the generated Javadoc files in '/home/richard/projects/envimate/opensource/message-mate/core/target/apidocs' dir

so the last resort is to assign a module name is to change the jar file name of the maven artifact.

As mentioned on https://blog.joda.org/2017/05/java-se-9-jpms-automatic-modules.html

The final missing piece of information is the module name. This has been a big point of discussion in Project Jigsaw. As it stands, the module name will be derived from the filename of the jar file. For me, this is a critical problem with the basic design of automatic modules (but see the mitigation section below).

SO we're left with the jar file name modification to avoid clashes.

This PR fixes that for this project.

This is no guarantee that loading the modules will succeed, but at the very least, it will enable me to go one step further, and see what the next problem is, if any.