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-mpi4py/package.py

@@ -0,0 +1,14 @@
+from spack import *
+
+class PyMpi4py(Package):
+    """This package provides Python bindings for the Message Passing Interface (MPI) standard. It is implemented on top of the MPI-1/MPI-2 specification and exposes an API which grounds on the standard MPI-2 C++ bindings."""
+    homepage = "https://pypi.python.org/pypi/mpi4py"
+    url      = "https://pypi.python.org/packages/source/m/mpi4py/mpi4py-1.3.1.tar.gz"
+
+    version('1.3.1', 'dbe9d22bdc8ed965c23a7ceb6f32fc3c')
+    extends('python')
+    depends_on('py-setuptools')
+    depends_on('mpi')
+
+    def install(self, spec, prefix):
+        python('setup.py', 'install', '--prefix=%s' % prefix)