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-rpy2/package.py

@@ -0,0 +1,17 @@
+from spack import *
+
+class PyRpy2(Package):
+    """rpy2 is a redesign and rewrite of rpy. It is providing a low-level interface to R from Python, a proposed high-level interface, including wrappers to graphical libraries, as well as R-like structures and functions."""
+    homepage = "https://pypi.python.org/pypi/rpy2"
+    url      = "https://pypi.python.org/packages/source/r/rpy2/rpy2-2.5.4.tar.gz"
+
+    version('2.5.4', '115a20ac30883f096da2bdfcab55196d')
+    version('2.5.6', 'a36e758b633ce6aec6a5f450bfee980f')
+
+    extends('python')
+    depends_on('py-setuptools')
+
+    depends_on('R')
+
+    def install(self, spec, prefix):
+        python('setup.py', 'install', '--prefix=%s' % prefix)