possible_dependencies() now reports missing dependencies

- Add an optional argument so that `possible_dependencies()` will report
  missing dependencies.
- Add a test to ensure it works.
- Ignore missing dependencies in `possible_dependencies()` by default.

var/spack/repos/builtin.mock/packages/missing-dependency/package.py

@@ -0,0 +1,21 @@
+# 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)
+
+from spack import *
+
+
+class MissingDependency(Package):
+    """Package with a dependency that does not exist."""
+
+    homepage = "http://www.example.com"
+    url      = "http://www.example.com/missing-dependency-1.0.tar.gz"
+
+    version('1.0', '0123456789abcdef0123456789abcdef')
+
+    # intentionally missing to test possible_dependencies()
+    depends_on("this-is-a-missing-dependency")
+
+    # this one is a "real" mock dependency
+    depends_on("a")