specs: remove parse_anonymous_spec(); use Spec() instead

- `parse_anonymous_spec()` is a vestige of the days when Spack didn't
  support nameless specs.  We don't need it anymore because now we can
  write Spec() for a spec that will match anything, and satisfies()
  semantics work properly for anonymous specs.

- Delete `parse_anonymous_spec()` and replace its uses with simple calls
  to the Spec() constructor.

- make then handling of when='...' specs in directives more consistent.

- clean up Spec.__contains__()

- refactor directives and tests slightly to accommodate the change.

var/spack/repos/builtin.mock/packages/when-directives-false/package.py

@@ -0,0 +1,28 @@
+# 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 WhenDirectivesFalse(Package):
+    """Package that tests False when specs on directives."""
+
+    homepage = "http://www.example.com"
+    url = "http://www.example.com/example-1.0.tar.gz"
+
+    version('1.0', '0123456789abcdef0123456789abcdef')
+
+    patch('https://example.com/foo.patch',
+          sha256='abcd1234abcd1234abcd1234abcd1234abcd1234abcd1234abcd1234abcd1234',
+          when=False)
+    extends('extendee', when=False)
+    depends_on('b', when=False)
+    conflicts('@1.0', when=False)
+    resource(url="http://www.example.com/example-1.0-resource.tar.gz",
+             md5='0123456789abcdef0123456789abcdef',
+             when=False)
+
+    def install(self, spec, prefix):
+        pass

var/spack/repos/builtin.mock/packages/when-directives-true/package.py

@@ -0,0 +1,28 @@
+# 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 WhenDirectivesTrue(Package):
+    """Package that tests True when specs on directives."""
+
+    homepage = "http://www.example.com"
+    url = "http://www.example.com/example-1.0.tar.gz"
+
+    version('1.0', '0123456789abcdef0123456789abcdef')
+
+    patch('https://example.com/foo.patch',
+          sha256='abcd1234abcd1234abcd1234abcd1234abcd1234abcd1234abcd1234abcd1234',
+          when=True)
+    extends('extendee', when=True)
+    depends_on('b', when=True)
+    conflicts('@1.0', when=True)
+    resource(url="http://www.example.com/example-1.0-resource.tar.gz",
+             md5='0123456789abcdef0123456789abcdef',
+             when=True)
+
+    def install(self, spec, prefix):
+        pass