Weld SE - manually registered extensions should reside in synthetic bean archive

Currently, portable extensions registered via Weld.addExtension end up in a separate bean archive (with suffix .additionalClasses). This is mostly fine but there are rare scenarios with per-bean-archive enablements where it might cause issues.

We could instead register them in the synthetic archive with all other manually added components.

I have no idea what was the initial idea behind this form of registration and if this was an oversight of an intention. Such extensions even come with a specific synthetic prefix to mark them.