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-mx/package.py

@@ -0,0 +1,13 @@
+from spack import *
+
+class PyMx(Package):
+    """The eGenix.com mx Base Distribution for Python is a collection of professional quality software tools which enhance Python's usability in many important areas such as fast text searching, date/time processing and high speed data types."""
+    homepage = "http://www.egenix.com/products/python/mxBase/"
+    url      = "https://downloads.egenix.com/python/egenix-mx-base-3.2.8.tar.gz"
+
+    version('3.2.8', '9d9d3a25f9dc051a15e97f452413423b')
+
+    extends('python')
+
+    def install(self, spec, prefix):
+        python('setup.py', 'install', '--prefix=%s' % prefix)