concretizer: require at least a dependency type to say the dependency holds

fixes #20784

Similarly to the previous bug, here we were deducing
conditions to be imposed on nodes that were not part
of the DAG.

var/spack/repos/builtin.mock/packages/test-dep-with-imposed-conditions/package.py

@@ -0,0 +1,17 @@
+# 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 TestDepWithImposedConditions(Package):
+    """Simple package with no dependencies"""
+
+    homepage = "http://www.example.com"
+    url = "http://www.example.com/e-1.0.tar.gz"
+
+    version('1.0', '0123456789abcdef0123456789abcdef')
+
+    depends_on('c@1.0', type='test')