qt-enterprise-linux-x64-5.6.0-rc_326: Cross-module documentation links are broken

All Many of the cross-module links in the packaged documentation seem to be broken.

In practice, what this means is that qdoc does not have access to the .index files for other modules when building the docs. For example, when building Qt Core docs, the Qt Reference Documentation (qtdoc repo) docs (and others) should already have been built in prepare mode (make prepare_docs), as that generates the index file.

In the attached image (Qt Quick landing page), two links are broken: Dialogs (Qt Quick Dialogs module) and 'QML Applications' (Qt Doc) are broken.

This issue cannot be reproduced when building the documentation locally using 'make docs' on the top-level qt5 repository.