concretizer: simplify "fact" method (#21148)

The "fact" method before was dealing with multiple facts
registered per call, which was used when we were emitting
grounded rules from knowledge of the problem instance.

Now that the encoding is changed we can simplify the method
to deal only with a single fact per call.

lib/spack/spack/solver/asp.py

@@ -210,19 +210,6 @@ def print_cores(self):
                 *sorted(str(symbol) for symbol in core))
 
 
-def _normalize(body):
-    """Accept an AspAnd object or a single Symbol and return a list of
-    symbols.
-    """
-    if isinstance(body, clingo.Symbol):
-        args = [body]
-    elif hasattr(body, 'symbol'):
-        args = [body.symbol()]
-    else:
-        raise TypeError("Invalid typee: ", type(body))
-    return args
-
-
 def _normalize_packages_yaml(packages_yaml):
     normalized_yaml = copy.copy(packages_yaml)
     for pkg_name in packages_yaml:
@@ -280,19 +267,14 @@ def newline(self):
 
     def fact(self, head):
         """ASP fact (a rule without a body)."""
-        symbols = _normalize(head)
-        self.out.write("%s.\n" % ','.join(str(a) for a in symbols))
+        symbol = head.symbol() if hasattr(head, 'symbol') else head
 
-        atoms = {}
-        for s in symbols:
-            atoms[s] = self.backend.add_atom(s)
+        self.out.write("%s.\n" % str(symbol))
 
-        self.backend.add_rule(
-            [atoms[s] for s in symbols], [], choice=self.cores
-        )
+        atom = self.backend.add_atom(symbol)
+        self.backend.add_rule([atom], [], choice=self.cores)
         if self.cores:
-            for s in symbols:
-                self.assumptions.append(atoms[s])
+            self.assumptions.append(atom)
 
     def solve(
             self, solver_setup, specs, dump=None, nmodels=0,