concretizer: remove ad-hoc rule for external packages (#20193)

fixes #20040

Matching compilers among nodes has been prioritized
in #20020. Selection of default variants has been
tuned in #20182. With this setup there is no need
to have an ad-hoc rule for external packages. On
the contrary it should be removed to prefer having
default variant values over more external nodes in
the DAG.

var/spack/repos/builtin.mock/packages/vdefault-or-external-root/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)
+class VdefaultOrExternalRoot(Package):
+    """Test that we don't prefer adding an external to using
+    a default variant value.
+    """
+
+    homepage = 'https://www.example.org'
+    url = 'https://example.org/files/v3.4/cmake-3.4.3.tar.gz'
+
+    version('1.0', '4cb3ff35b2472aae70f542116d616e63')
+
+    depends_on('vdefault-or-external')

var/spack/repos/builtin.mock/packages/vdefault-or-external/package.py

@@ -0,0 +1,17 @@
+# 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)
+class VdefaultOrExternal(Package):
+    """Test that we don't prefer adding an external to using
+    a default variant value.
+    """
+
+    homepage = 'https://www.example.org'
+    url = 'https://example.org/files/v3.4/cmake-3.4.3.tar.gz'
+
+    version('1.0', '4cb3ff35b2472aae70f542116d616e63')
+
+    variant('external', default=False, description='nope')
+
+    depends_on('externaltool', when='+external')