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-dateutil/package.py

@@ -0,0 +1,16 @@
+from spack import *
+
+class PyDateutil(Package):
+    """Extensions to the standard Python datetime module."""
+    homepage = "https://pypi.python.org/pypi/dateutil"
+    url      = "https://pypi.python.org/packages/source/p/python-dateutil/python-dateutil-2.4.0.tar.gz"
+
+    version('2.4.0', '75714163bb96bedd07685cdb2071b8bc')
+    version('2.4.2', '4ef68e1c485b09e9f034e10473e5add2')
+
+    extends('python')
+    depends_on('py-setuptools')
+    depends_on('py-six')
+
+    def install(self, spec, prefix):
+        python('setup.py', 'install', '--prefix=%s' % prefix)