Extension Manager special optional behavior should be limited to XWiki extensions

We decide a while ago to reuse Maven dependencies optional property to indicate that a dependency should be installed by default but can be uninstalled without uninstalling the backward dependency.

The only deference was supposed to be the fact that's it's installed by default, unfortunately many project on Maven central (badly) use optional for dependencies which are only needed at build time and are totally useless at runtime (sometimes those dependencies don't even exist on Maven central). This makes XWiki install way to much stuff sometimes which is not great.