qdoc: No documentation generated for classes under a \namespace declared in another module.

QDoc has the ability to resolve namespaces declared in another module, but this only works in single-exec mode. Namespaces read from index trees are currently ignored.

This means, for example, that with 'make docs', classes under the Qt3D namespace (documented in Qt3DCore) are correctly documented in Qt3DCore only. In Qt3DRenderer, QDoc does not 'see' the Qt3D namespace and all class documentation gets skipped.

Single-exec mode does not have this issue.