Rewrite of JAR plugin ready for merge

Hello all

A pull request which rewrites the Maven JAR Plugin is ready for merge 
and already got some reviews:

    https://github.com/apache/maven-jar-plugin/pull/508

It has been discussed through some channels (GitHub, Slack). This email 
is for bringing attention via another channel in order to see if there 
is objection to the merge.

The major insensitive for the rewrite was to support Java Modules Source 
Hierarchy. This is essential for continuing progress on the Java module 
support. Internally, the major change is that the new implementation 
uses the `jar` tool bundled in the JDK instead of our own Maven Archiver 
(except for generating the `MANIFEST.MF` content, where Maven Archiver 
is still used). It provides advantage such as safer multi-release JAR 
files and enable the use of more options in the future such as module 
hashes.

If there is any comment or objection, please let us known. If none, I 
propose that we merge later this week.

Thanks,

     Martin