Strict dependency constraint checking

[scheibelp]

Satisfaction checks on dependencies should be strict when the parent check is strict

This is intended for constraints like

depends_on('py-backports-functools-lru-cache', when='^python@:2')

currently, Spack will add the py-backports-functools-lru-cache dependency at a point in the concretization when the dependency is just ^python (without a version) because strict constraint checking does not propagate to dependency constraint checking.

Note that because the concretizer is greedy, the constraints applied by the py-backports-functools-lru-cache dependency will be applied later and may generate concretizer conflicts (which requires the user to specify certain constraints on the command line), but this avoids the concretizer incorrectly adding the py-backports-functools-lru-cache dependency.

An example package this fixes is py-soupsieve.