Seamless translation from 'target=<generic>' to either
- target.family == <generic> (in methods)
- 'target=<generic>:' (in directives)
Also updated docs to show ranges in directives.
- 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
