concretizer: handle virtual spec constraints better

Specs like hdf5 ^mpi were unsatisfiable because we added a requierment
for `node("mpi").`.  This can't be resolved because "mpi" is not a
package.

- [x] Introduce `virtual_node()`, which says *some* provider must be in
      the DAG.

lib/spack/spack/solver/asp.py

@@ -360,7 +360,7 @@ def pkg_rules(self, pkg):
                 self.fact(
                     fn.variant_default_value(pkg.name, name, variant.default))
             else:
-                self.rule(self._not(single), fn.node(pkg.name))
+                self.fact(self._not(single))
                 defaults = variant.default.split(',')
                 for val in sorted(defaults):
                     self.fact(fn.variant_default_value(pkg.name, name, val))
@@ -706,9 +706,12 @@ def generate_asp_program(self, specs):
             self.fact(fn.root(spec.name))
             for dep in spec.traverse():
                 self.h2('Spec: %s' % str(dep))
-                self.fact(fn.node(dep.name))
-                for clause in self.spec_clauses(dep):
-                    self.rule(clause, fn.node(dep.name))
+
+                if dep.virtual:
+                    self.fact(fn.virtual_node(dep.name))
+                else:
+                    for clause in self.spec_clauses(dep):
+                        self.fact(clause)
 
         self.out.write('\n')
         display_lp = pkgutil.get_data('spack.solver', 'display.lp')

lib/spack/spack/solver/concretize.lp

@@ -33,6 +33,9 @@ depends_on(P, D, T) :- declared_dependency(P, D, T), not virtual(D), node(P).
 1 { depends_on(P, Q, T) : provides_virtual(Q, V) } 1
     :- declared_dependency(P, V, T), virtual(V), node(P).
 
+% if a virtual was required by some root spec, one provider is in the DAG
+1 { node(P) : provides_virtual(P, V) } 1 :- virtual_node(V).
+
 % for any virtual, there can be at most one provider in the DAG
 provider(P, V) :- node(P), provides_virtual(P, V).
 0 { provider(P, V) : node(P) } 1 :- virtual(V).
@@ -66,6 +69,7 @@ node(D) :- node(P), depends_on(P, D).
 #defined depends_on/3.
 #defined declared_dependency/3.
 #defined virtual/1.
+#defined virtual_node/1.
 #defined provides_virtual/2.
 #defined pkg_provider_preference/4.
 #defined default_provider_preference/3.