Draft: COMP: Fix and update build-system to streamline maintenance & reuse

• Ensure inner-build doesn't override the outer build by setting the EXTENSION_BUILD_SUBDIRECTORY variable specific to the SuperBuild extension layout.

• Update comments to match existing convention

• Simplify download of vespa sources:

  • Use a single FetchContent_Populate
  • Introduce vespa_SOURCE_DIR variable
  • Improve comment describing integration

• Speed-up build downloading GCAL release archive

• Update and rename Eigen project to Eigen3

  • Also speed-up build downloading release archive
  • Adapted from https://gitlab.kitware.com/aeva/slicersmtk/-/blob/master/SuperBuild/External_Eigen3.cmake
  • Set CMAKE_FIND_PACKAGE_PREFER_CONFIG to ensure that Eigen3 config file is used instead of older "FindEigen3.cmake" provided by CGAL by set.

• Add dedicated section for each library being explicitly packaged

• Fix setting of CLIs RPATH properties to ensure they are set only on Linux