Rewrite CMake configuration in "modern CMake"

Rewrite of LHCb CMake configuration along the lines of gaudi/Gaudi!922 (closed) (and gaudi/Gaudi!986 (merged)).

The main changes are:

• public headers moved into include directories
• all external dependencies collected in the cmake/LHCbDependencies.cmake file
• removed *Sys packages
• cleaned up the internal dependencies (link only what is actually needed)

It requires

• gaudi/Gaudi!1177 (merged)
• lhcb-core/LbDevTools!167 (merged)
• lhcb-core/LbNightlyTools!333 (merged)
• Detector!73 (merged)
• lhcb-core/LbDevTools!169 (merged)
• lhcb-core/LbEnv!114 (merged)
• lhcb-core/LbDevTools!171 (merged)

Downstream projects require a few backward compatible fixes due to a more strict configuration in LHCb:

• Lbcom!544 (merged)
• Online!481 (merged)
• Rec!2410 (merged)
• Phys!927 (merged)
• Allen!566 (merged)
• Alignment!180 (merged)
• Panoptes!119 (merged)

Missing bits

• custom environment(?)
• refactoring
• Detector/DD4hep support
• move the remaining 2 local dependencies to the global deps file