Export external dependencies CMake variables required by "find_package()" into vtk-config associated with VTK build-tree

To drastically streamline the configuration of project indirectly depending on VTK build tree having module like OpenVR enabled, including the relevant CMake variables in the vtk-config associated with the build tree should be implemented.

Two cases:

• For project providing a <name>-config.cmake file, the variable <name>_DIR should be included
• For project with a Find<name>.cmake module shipped with VTK, the relevant <name>_INCLUDE_DIR, <name>_LIBRARY_NAME and <name>_OPTION1 should be exported.

Related issues:

• #18338
• #18328 (closed)