Old concretizer: prevent unexpected propagation of external config (#20976)

When using an external package with the old concretizer, all
dependencies of that external package were severed. This was not
performed bidirectionally though, so for an external package W with
a dependency on Z, if some other package Y depended on Z, Z could
still pull properties (e.g. compiler) from W since it was not
severed as a parent dependency.

This performs the severing bidirectionally, and adds tests to
confirm expected behavior when using config from DAG-adjacent
packages during concretization.

var/spack/repos/builtin.mock/packages/external-common-gdbm/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)
+
+from spack import *
+
+
+class ExternalCommonGdbm(Package):
+    homepage = "http://www.gnu.org.ua/software/gdbm/gdbm.html"
+    url      = "https://ftpmirror.gnu.org/gdbm/gdbm-1.18.1.tar.gz"
+
+    version('1.18.1', 'be78e48cdfc1a7ad90efff146dce6cfe')

var/spack/repos/builtin.mock/packages/external-common-openssl/package.py

@@ -0,0 +1,14 @@
+# 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)
+
+from spack import *
+
+
+class ExternalCommonOpenssl(Package):
+    homepage = "http://www.openssl.org"
+    url      = "http://www.openssl.org/source/openssl-1.1.1i.tar.gz"
+
+    version('1.1.1i', 'be78e48cdfc1a7ad90efff146dce6cfe')
+    depends_on('external-common-perl')

var/spack/repos/builtin.mock/packages/external-common-perl/package.py

@@ -0,0 +1,14 @@
+# 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)
+
+from spack import *
+
+
+class ExternalCommonPerl(Package):
+    homepage = "http://www.perl.org"
+    url      = "http://www.cpan.org/src/5.0/perl-5.32.0.tar.gz"
+
+    version('5.32.0', 'be78e48cdfc1a7ad90efff146dce6cfe')
+    depends_on('external-common-gdbm')

var/spack/repos/builtin.mock/packages/external-common-python/package.py

@@ -0,0 +1,15 @@
+# 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)
+
+from spack import *
+
+
+class ExternalCommonPython(Package):
+    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')
+    depends_on('external-common-openssl')
+    depends_on('external-common-gdbm')