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-pil/package.py

@@ -0,0 +1,14 @@
+from spack import *
+
+class PyPil(Package):
+    """The Python Imaging Library (PIL) adds image processing capabilities to your Python interpreter. This library supports many file formats, and provides powerful image processing and graphics capabilities."""
+
+    homepage = "http://www.pythonware.com/products/pil/"
+    url      = "http://effbot.org/media/downloads/Imaging-1.1.7.tar.gz"
+
+    version('1.1.7', 'fc14a54e1ce02a0225be8854bfba478e')
+
+    extends('python')
+
+    def install(self, spec, prefix):
+        python('setup.py', 'install', '--prefix=%s' % prefix)