RFC: COMP: use relative includes for VTK and ITK

Opening for discussion, ref https://discourse.slicer.org/t/duplicated-includes/1862

• script: https://gist.github.com/ihnorton/b9d0a18b3ae00bb5fb78b143eda79d70

• how to spot-check (in slicer source dir "s5"):

:s5 $ cat `find ./ -name *.cxx -or -name *.h` | egrep "#include.*itk" | less
:s5 $ cat `find ./ -name *.cxx -or -name *.h` | egrep "#include.*vtk" | less

• Build system TODO
  • Teach build system to clone {vtk,itk} in e.g. SuperBuild/VTKv9/vtk
  • Teach build system to pass single include path for vtk as e.g. SuperBuild/VTKv9/
  • ? Fix bundling cmakescript ?