Public fields in extensions should not be allowed

Since the container must provide an @ApplicationScoped bean for every extension and since a proxy must be injected into injection points requesting this bean, it should be clear that extensions should not be allowed to have public fields. The spec doesn't specify this explicitly, but it should - like it does for non-dependent managed beans.

The container should treat this as a definition error, otherwise people may end up accessing these public fields of extensions and getting weird errors.

See also WELD-1474