Externals are preferred even when they have non-default variant values

fixes #22596

Variants which are specified in an external spec are not
scored negatively if they encode a non-default value.

var/spack/repos/builtin.mock/packages/external-non-default-variant/package.py

@@ -0,0 +1,13 @@
+# 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 ExternalNonDefaultVariant(Package):
+    """An external that is registered with a non-default value"""
+    homepage = "http://www.python.org"
+    url = "http://www.python.org/ftp/python/3.8.7/Python-3.8.7.tgz"
+
+    version('3.8.7', 'be78e48cdfc1a7ad90efff146dce6cfe')
+
+    variant('foo', default=True, description='just a variant')
+    variant('bar', default=True, description='just a variant')

var/spack/repos/builtin.mock/packages/trigger-external-non-default-variant/package.py

@@ -0,0 +1,12 @@
+# 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 TriggerExternalNonDefaultVariant(Package):
+    """This ackage depends on an external with a non-default variant"""
+    homepage = "http://www.example.com"
+    url = "http://www.someurl.tar.gz"
+
+    version('1.0', 'foobarbaz')
+
+    depends_on('external-non-default-variant')