`conditional()`, which defines conditional variant values, and the other ways to declare
variant values should probably be in a layer above `spack.variant`. This does the simple
thing and moves *just* `conditional()` to `spack.directives` to avoid a circular import.
We can revisit the public variant interface later, when we split packages from core.
Co-authored-by: Harmen Stoppels <me@harmenstoppels.nl>
Signed-off-by: Todd Gamblin <tgamblin@llnl.gov>
6961514122