The reason being that this way Maven will force the creation of so called automatic modules out of Weld dependencies

It's not maven which create this it's the java compiler because maven called it witht the jar on the module path.

Also, I cannot really say if this would be a blocker if we were to modularize Weld (which I unsure we aim to, at least any time soon TM), but it might prove to be so.

It will. You can double check by going to the openJDK mailing list as I have done and ask about package splitting.
Honnestly I don't understand why the question of supporting modularity is asked, when support Java EE 8 servers will be released, Java 9 will be every where and not supporting it might be seen as being legacy.

Fixing this problem is mainly deciding to move three classes in a new sub-package. It may break backward compatibility, hat's why we should introduce it in a major release: Weld 3.0.0