ASP-based solver: reordered low priority optimization criteria (#24184)

Minimizing compiler mismatches in the DAG and preferring newer 
versions of packages are now higher priority than trying to use as 
many default values as possible in multi-valued variants.

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

@@ -0,0 +1,10 @@
+# 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 Gmt(Package):
+
+    version('2.0', 'abcdef')
+    version('1.0', 'abcdef')
+
+    depends_on('mvdefaults', when='@1.0')

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

@@ -0,0 +1,10 @@
+# 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 Mvdefaults(Package):
+
+    version('1.0', 'abcdef')
+
+    variant('foo', values=('a', 'b', 'c'), default=('a', 'b', 'c'),
+            multi=True, description='')

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

@@ -0,0 +1,8 @@
+# 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 Root(Package):
+    version('1.0', 'abcdef')
+
+    depends_on('gmt')