concretize.lp: impose a lower bound on the number of version facts if a solution exists (#31142)

* concretize.lp: impose a lower bound on the number of version facts if a valid version exists

fixes #30864

* Add a unit test

var/spack/repos/builtin.mock/packages/non-existing-conditional-dep/package.py

@@ -0,0 +1,16 @@
+# Copyright 2013-2022 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)
+from spack.package import *
+
+
+class NonExistingConditionalDep(Package):
+    """Simple package with no source and one dependency"""
+
+    homepage = "http://www.example.com"
+
+    version('2.0')
+    version('1.0')
+
+    depends_on('dep-with-variants@999', when='@2.0')