concretizer: use cardinality constraints for versions

Instead of python callbacks, use cardinality constraints for package
versions. This is slightly faster and has the advantage that it can be
written to an ASP program to be executed *outside* of Spack. We can use
this in the future to unify the pyclingo driver and the clingo text
driver.

This makes use of add_weight_rule() to implement cardinality constraints.
add_weight_rule() only has a lower bound parameter, but you can implement
a strict "exactly one of" constraint using it. In particular, wee want to
define:

    1 {v1; v2; v3; ...} 1 :- version_satisfies(pkg, constraint).
    version_satisfies(pkg, constraint) :- 1 {v1; v2; v3; ...} 1.

And we do that like this, for every version constraint:

    atleast1(pkg, constr) :- 1 {version(pkg, v1); version(pkg, v2); ...}.
    morethan1(pkg, constr) :- 2 {version(pkg, v1); version(pkg, v2); ...}.
    version_satisfies(pkg, constr) :- atleast1, not morethan1(pkg, constr).
    :- version_satisfies(pkg, constr), morethan1.
    :- version_satisfies(pkg, constr), not atleast1.

v1, v2, v3, etc. are computed on the Python side by comparing every
possible package version with the constraint.

Computing things like this has the added advantage that if v1, v2, v3,
etc. comprise *all* possible versions of a package, we can just omit the
rules for the constraint under consideration. This happens pretty
frequently in the Spack mainline.

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