concretizer: each external version is allowed by definition (#20247)

Registering external versions among the lists of allowed ones
generates the correct rules for `version_satisfies`

var/spack/repos/builtin.mock/packages/external-buildable-with-variant/package.py

@@ -0,0 +1,15 @@
+# Copyright 2013-2020 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 import *
+
+
+class ExternalBuildableWithVariant(Package):
+    homepage = "http://somewhere.com"
+    url = "http://somewhere.com/module-1.0.tar.gz"
+
+    version('1.0', '1234567890abcdef1234567890abcdef')
+
+    variant('baz', default=False, description='nope')

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

@@ -12,5 +12,6 @@ class Externaltool(Package):
 
     version('1.0', '1234567890abcdef1234567890abcdef')
     version('0.9', '1234567890abcdef1234567890abcdef')
+    version('0.8.1', '1234567890abcdef1234567890abcdef')
 
     depends_on('externalprereq')