REG: QDoc never links using relative hrefs when running in single-exec mode

Consider a documentation build consisting of multiple projects (top-level .qdocconf files), with each generating output to a different output subdirectory; that is, a typical multi-module build such as Qt framework docs in offline mode where output subdirectories are in use.

When running in single-exec mode, QDoc generates links between entities as if they are located in a single, shared output directory. The links are missing the target's output subdirectory component.