Added a function that concretizes specs together (#11158)

* Added a function that concretizes specs together

* Specs concretized together are copied instead of being referenced

This makes the specs different objects and removes any reference to the
fake root package that is needed currently for concretization.

* Factored creating a repository for concretization into its own function

* Added a test on overlapping dependencies

share/spack/templates/misc/coconcretization.pyt

@@ -0,0 +1,15 @@
+# Copyright 2013-2019 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 Concretizationroot(Package):
+    url = 'fake_url'
+
+    version('1.0')
+
+{% for dep in specs %}
+    depends_on('{{ dep }}')
+{% endfor %}
+