qdoc: Cannot link to externally-built projects via .index files

Cross-linking between documentation sets works as long as all documentation sets (modules) are part of the same build process. Trying to pull in .index files from externally-built add-ons or others projects work only if -installdir <path> option was not used. In this case, cross-linking is always done using the full URLs (https://...) - but this is not optimal as most files are located in the same output directory (structure).

We need to detect whether an index file being loaded is part of the current documentation build process or something external, and if so, use full URLs to form links to entities from that index file. The 'url' attribute of the index XML is suitable for this.