- 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 '--test=all' and '--test=root' options to test either the root or the root and all dependencies.
* add a test dependency type that is only used when --test is enabled.
* test dependencies are not added to the spec, but they are provided in the test environment.
