concretizer: move conditional dependency logic into `concretize.lp`

Continuing to convert everything in `asp.py` into facts, make the
generation of ground rules for conditional dependencies use facts, and
move the semantics into `concretize.lp`.

This is probably the most complex logic in Spack, as dependencies can be
conditional on anything, and we need conditional ASP rules to accumulate
and map all the dependency conditions to spec attributes.

The logic looks complicated, but essentially it accumulates any
constraints associated with particular conditions into a fact associated
with the condition by id. Then, if *any* condition id's fact is True, we
trigger the dependency.

This simplifies the way `declared_dependency()` works -- the dependency
is now declared regardless of whether it is conditional, and the
conditions are handled by `dependency_condition()` facts.

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