Rewrite CMake configuration in "modern CMake"

Rewrite of the CMake configuration along the lines of LHCb!2931 (merged) (see there for more details).

Requires Lbcom!551 (merged), LHCb!2931 (merged), Rec!2416 (merged) and Phys!931 (merged)

Notes

• I modified Calibration/Pi0Calibration to avoid the silly rebuild of all sources (once as a library and once as a module)
• I moved a misplaced Python file in Calibration/Pi0Calibration because the ConfigurableUser class in there was never seen by the build
• AlignmentSys is empty, but I kept it as is is used by https://gitlab.cern.ch/lhcb-core/ProjectsDocPages/-/blob/master/scripts/addrel.py