- remove the old LGPL license headers from all files in Spack
- add SPDX headers to all files
- core and most packages are (Apache-2.0 OR MIT)
- a very small number of remaining packages are LGPL-2.1-only
* add OctavePackage
1. remove import CudaPackage which is not needed anymore
2. mention CudaPackage and OctavePackage in packaging guide
3. adjust OctavePackageTemplate
4. add clue file for Octave build
5. sanity check on self.prefix
* use setup_environment
