API change review CMake parser needs to be smarter.

When we moved away from using sync.profile perl fragments to record the public headers, I wrote a quick and dirty parser, qtqa/scripts/api-review/cmakescan.py, to extract that information from our CMake config. It assumed, simplistically, that each Qt module was created by a (unique) qt_internal_add_*_module() call that contained all the information we need.

It turns out that qtgraphs has more sophisticated CMake config: src/CMakeLists.txt has various features to take into account, that switch it between different places that it calls qt_internal_add_qml_module(Graphs …), after which it goes on to some add_subdirectory() calls and those subdirectories' CMakeLists.txt may qt_internal_extend_target(Graphs …) which may extend the list of relevant sources. Of course, there may be other modules that do similar, especially the subdirectories.

The parser thus needs to be a bit more sophisticated: in particular, it needs to notice add_subdirectory() and actually scan the CMakeLists.txt files of those, recursively, for additions to the module.
Where there are several qt_internal_add_*_module() for the same Qt module, the list of sources should, of course, be the union of those from all calls.