new command: spack maintainers queries package maintainers

- We don't currently make enough use of the maintainers field on
  packages, though we could use it to assign reviews.

- add a command that allows maintainers to be queried
  - can ask who is maintaining a package or packages
  - can ask what packages users are maintaining
  - can list all maintained or unmaintained packages

- add tests for the command

var/spack/repos/builtin.mock/packages/maintainers-1/package.py

@@ -0,0 +1,20 @@
+# 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 Maintainers1(Package):
+    """Package with a maintainers field."""
+
+    homepage = "http://www.example.com"
+    url      = "http://www.example.com/maintainers-1.0.tar.gz"
+
+    maintainers = ['user1', 'user2']
+
+    version('1.0', '0123456789abcdef0123456789abcdef')
+
+    def install(self, spec, prefix):
+        pass

var/spack/repos/builtin.mock/packages/maintainers-2/package.py

@@ -0,0 +1,20 @@
+# 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 Maintainers2(Package):
+    """A second package with a maintainers field."""
+
+    homepage = "http://www.example.com"
+    url      = "http://www.example.com/maintainers2-1.0.tar.gz"
+
+    maintainers = ['user2', 'user3']
+
+    version('1.0', '0123456789abcdef0123456789abcdef')
+
+    def install(self, spec, prefix):
+        pass