New, cleaner package repository structure.

Package repositories now look like this:

    top-level-dir/
        repo.yaml
        packages/
            libelf/
                package.py
            mpich/
                package.py
            ...

This leaves room at the top level for additional metadata, source,
per-repo configs, indexes, etc., and it makes it easy to see that
something is a spack repo (just look for repo.yaml and packages).

var/spack/repos/builtin.mock/packages/multimethod/package.py

@@ -0,0 +1,143 @@
+##############################################################################
+# Copyright (c) 2013, Lawrence Livermore National Security, LLC.
+# Produced at the Lawrence Livermore National Laboratory.
+#
+# This file is part of Spack.
+# Written by Todd Gamblin, tgamblin@llnl.gov, All rights reserved.
+# LLNL-CODE-647188
+#
+# For details, see https://scalability-llnl.github.io/spack
+# Please also see the LICENSE file for our notice and the LGPL.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License (as published by
+# the Free Software Foundation) version 2.1 dated February 1999.
+#
+# This program is distributed in the hope that it will be useful, but
+# WITHOUT ANY WARRANTY; without even the IMPLIED WARRANTY OF
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the terms and
+# conditions of the GNU General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public License
+# along with this program; if not, write to the Free Software Foundation,
+# Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
+##############################################################################
+from spack import *
+
+
+class Multimethod(Package):
+    """This package is designed for use with Spack's multimethod test.
+       It has a bunch of test cases for the @when decorator that the
+       test uses.
+    """
+
+    homepage = 'http://www.example.com/'
+    url      = 'http://www.example.com/example-1.0.tar.gz'
+
+    #
+    # These functions are only valid for versions 1, 2, and 3.
+    #
+    @when('@1.0')
+    def no_version_2(self):
+        return 1
+
+    @when('@3.0')
+    def no_version_2(self):
+        return 3
+
+    @when('@4.0')
+    def no_version_2(self):
+        return 4
+
+
+    #
+    # These functions overlap, so there is ambiguity, but we'll take
+    # the first one.
+    #
+    @when('@:4')
+    def version_overlap(self):
+        return 1
+
+    @when('@2:')
+    def version_overlap(self):
+        return 2
+
+
+    #
+    # More complicated case with cascading versions.
+    #
+    def mpi_version(self):
+        return 0
+
+    @when('^mpi@3:')
+    def mpi_version(self):
+        return 3
+
+    @when('^mpi@2:')
+    def mpi_version(self):
+        return 2
+
+    @when('^mpi@1:')
+    def mpi_version(self):
+        return 1
+
+
+    #
+    # Use these to test whether the default method is called when no
+    # match is found.  This also tests whether we can switch methods
+    # on compilers
+    #
+    def has_a_default(self):
+        return 'default'
+
+    @when('%gcc')
+    def has_a_default(self):
+        return 'gcc'
+
+    @when('%intel')
+    def has_a_default(self):
+        return 'intel'
+
+
+
+    #
+    # Make sure we can switch methods on different architectures
+    #
+    @when('=x86_64')
+    def different_by_architecture(self):
+        return 'x86_64'
+
+    @when('=ppc64')
+    def different_by_architecture(self):
+        return 'ppc64'
+
+    @when('=ppc32')
+    def different_by_architecture(self):
+        return 'ppc32'
+
+    @when('=arm64')
+    def different_by_architecture(self):
+        return 'arm64'
+
+
+    #
+    # Make sure we can switch methods on different dependencies
+    #
+    @when('^mpich')
+    def different_by_dep(self):
+        return 'mpich'
+
+    @when('^zmpi')
+    def different_by_dep(self):
+        return 'zmpi'
+
+
+    #
+    # Make sure we can switch on virtual dependencies
+    #
+    def different_by_virtual_dep(self):
+        return 1
+
+    @when('^mpi@2:')
+    def different_by_virtual_dep(self):
+        return 2