- 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
Fixes#4898
Constraints that were supposed to be conditionally activated for
specified values of a single-valued variant were being activated
unconditionally in the case that the variant was associated with
an implicit dependency. For example if X->Y->Z and Y places a
conditional constraint on Z for a given single-valued variant on
Y, then it would have been applied unconditionally when
concretizing X.
