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/packages/py-mock/package.py

@@ -0,0 +1,17 @@
+from spack import *
+
+class PyMock(Package):
+    """mock is a library for testing in Python. It allows you to replace parts
+    of your system under test with mock objects and make assertions about how
+    they have been used."""
+
+    homepage = "https://github.com/testing-cabal/mock"
+    url      = "https://pypi.python.org/packages/source/m/mock/mock-1.3.0.tar.gz"
+
+    version('1.3.0', '73ee8a4afb3ff4da1b4afa287f39fdeb')
+
+    extends('python')
+    depends_on('py-setuptools@17.1:')
+
+    def install(self, spec, prefix):
+        python('setup.py', 'install', '--prefix=%s' % prefix)