Jetty distribution does not automatically bundle webjars used by XAR applications

If you add a webjar dependency on a XAR application (for instance: xwiki-platform-panels-ui), it will not be bundled in the Jetty distribution, unless you manually add it to the war.

That is why we have these dependencies:
https://github.com/xwiki/xwiki-enterprise/blob/master/xwiki-enterprise-web/pom.xml#L851

Which is not very good IMO. (see comments there: https://github.com/xwiki/xwiki-enterprise/commit/0b544af38da866cdfe9eb570370b23d4a925649e)