scriptingbundle-maven-plugin: Move "metadata" mojo to another default phase

Currently the "metadata" mojo is bound to phase "prepare-package".
That does not work together with the bnd-maven-plugin which generates all metadata in phase "process-classes" (https://github.com/bndtools/bnd/tree/master/maven/bnd-maven-plugin#bnd-process-goal).

Instead I propose the following default phase "process-classes" or "compile".
Unfortunately it will share the phase with either the htl-maven-plugin or the bnd-maven-plugin.

This becomes obsolete once SLING-9776 is implemented.