ASP-based solver: model disjoint sets for multivalued variants (#22534)

* ASP-based solver: avoid adding values to variants when they're set

fixes #22533
fixes #21911

Added a rule that prevents any value to slip in a variant when the
variant is set explicitly. This is relevant for multi-valued variants,
in particular for those that have disjoint sets of values.

* Ensure disjoint sets have a clear semantics for external packages

var/spack/repos/builtin.mock/packages/mvapich2/package.py

@@ -0,0 +1,15 @@
+# Copyright 2013-2021 Lawrence Livermore National Security, LLC and other
+# Spack Project Developers. See the top-level COPYRIGHT file for details.
+#
+# SPDX-License-Identifier: (Apache-2.0 OR MIT)
+class Mvapich2(Package):
+    homepage = "http://www.homepage.org"
+    url = "http://www.someurl"
+
+    version('1.5', '9c5d5d4fe1e17dd12153f40bc5b6dbc0')
+
+    variant(
+        'file_systems',
+        description='List of the ROMIO file systems to activate',
+        values=auto_or_any_combination_of('lustre', 'gpfs', 'nfs', 'ufs'),
+    )