[FIX] Allow multiple namespaces if available

Some modules don't support multiple namespaces (for example
query+allpages) but it's possible that the generator factory sets it to
multiple namespaces. This also emits a warning if the namespace is
defined for a generator which does not support it.

Bug: T76794
Change-Id: I486d06fa6e03692f7c05725b22319e192b5d62c2
Merge "[FIX] Allow multiple namespaces if available"