Tracking/Acts: cmake cleanup

Frank Winklmeierrequested to merge
fwinkl/athena:cmake_acts into master

Cleanup the cmake configuration of the Tracking/Acts packages:

  • remove atlas_depends_on_subdirs
  • fix external package dependencies
  • remove unused ActsPriVtxFinderLib
  • move private components headers to src directory
  • enable flake8 where applicable and fix code

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