CMake: Make sure the Graphs2DImpl package dependency is recorded

Record the Graphs2DImpl dependency and create a package look up for it
in the module's Dependencies.cmake file.

Without the namespace, the recording is not done.

Amends 6adf6a4b4668dc817cf7e8c2c582b1a46c5e0de4

Pick-to: 6.12
Fixes: QTBUG-151626
Change-Id: Idaa8ac5438ff5a89e92ddd6f9b7ef949be333876
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>