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-gnuplot/package.py

@@ -0,0 +1,14 @@
+from spack import *
+
+class PyGnuplot(Package):
+    """Gnuplot.py is a Python package that allows you to create graphs from within Python using the gnuplot plotting program."""
+    homepage = "http://gnuplot-py.sourceforge.net/"
+    url      = "http://downloads.sourceforge.net/project/gnuplot-py/Gnuplot-py/1.8/gnuplot-py-1.8.tar.gz"
+
+    version('1.8', 'abd6f571e7aec68ae7db90a5217cd5b1')
+
+    extends('python')
+    depends_on('py-numpy')
+
+    def install(self, spec, prefix):
+        python('setup.py', 'install', '--prefix=%s' % prefix)