Tweaks to the Maven and OSGi build files

  * the `maven-dist` target now copies over the jar built from the `jar` target
    (instead of re-packaging from scratch). This notably means the maven
    artifact will contain the OSGi metadata added by the `jar` target.
  * in the OSGi bnd configuration file (used to generate OSGi metadata), only
    the required packages are explicitly listed, all the other package dependencies
    are set as optional. This notably means that all the proprietary libs in
    the classpath of the `compile-everything` target will be declared as optional
    dependencies in the OSGi manifest.
  * in the Maven POM, updated Saxon's version to 9.4.0.7.
  * in the Maven POM, made the Jing and TagSoup dependencies optional
  * in the Maven POM, added an optional dependency for Henri Sivonen's
    html-parser (back in Maven since version 1.4)
  * in the Maven POM, added optional dependencies for javax.mail and
    xmlunit.