concretizer: treat target ranges in directives correctly (#19988)

fixes #19981

This commit adds support for target ranges in directives,
for instance:

conflicts('+foo', when='target=x86_64:,aarch64:')

If any target in a spec body is not a known target the
following clause will be emitted:

node_target_satisfies(Package, TargetConstraint)

when traversing the spec and a definition of
the clause will then be printed at the end similarly
to what is done for package and compiler versions.

var/spack/repos/builtin.mock/packages/impossible-concretization/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 ImpossibleConcretization(Package):
+    """Package that should be impossible to concretize due to a conflict
+    with target ranges. See Issue 19981.
+    """
+
+    homepage = "http://www.example.com"
+    url = "http://www.example.com/example-1.0.tar.gz"
+
+    version(1.0, 'foobarbaz')
+
+    conflicts('target=x86_64:')