concretizer: don't require a provider for virtual deps if spec is external

This commit introduces a new rule:

real_node(Package) :- not external(Package), node(Package).

that permits to distinguish between an external node and a
real node that shouldn't trim dependency. It solves the
case of concretizing ninja with an external Python.

var/spack/repos/builtin.mock/packages/requires-virtual/package.py

@@ -0,0 +1,16 @@
+# 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 RequiresVirtual(Package):
+    """Package that requires a virtual dependency and is registered
+    as an external.
+    """
+
+    homepage = "http://www.example.com"
+    url = "http://www.example.com/a-1.0.tar.gz"
+
+    version('2.0', '2.0_a_hash')
+
+    depends_on('stuff')