qt6-declarative qmlcachegen non-determinism

While working on reproducible builds for openSUSE, I found that
there is a problem in how cmake is used with qt6-declarative's qmlcachegen.
I had filed QTBUG-121643 earlier but it seems much of its complexity was not resolved back then. We still have at least 5 packages that vary in their machine-code (unless building with -j1)
qt6-datavis3d
qt6-declarative
qt6-quick3d
qt6-sensors
qt6-virtualkeyboard
The underlying problem seems to be that qmlcachegen processes a lot of generated input files and if cmake does not know about all of these, it will happily run qmlcachegen earlier in the build process when only part of the inputs are present, which results in smaller output, and no errors.

Somehow it should be ensured that qmlcachegen output is deterministic. That should be the case if qmlcachegen is run after all its inputs are present.