cmake INTERFACE lib: "file is not part of any project"

Ahoj!

I've noticed some glitches in developing cmake' INTERFACE library using QC, in particular:

1. if CMakeLists.txt contains only INTERFACE target then its files always marked as "not part of any project" with many-many errors (see qc-interface-bug1.png)
2. if CMakeLists.txt additionally contains target which uses the interface library then it becomes a little better – files of the library marked as "not part of any project" only before manually rerunning cmake (qc-interface-bug2.{1,2}.png correspondingly).
3. if interface library uses target_sources then in case 2 everything works. I'd love to use this case but only after #15677 will be fixed.