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-pmw/package.py

@@ -0,0 +1,13 @@
+from spack import *
+
+class PyPmw(Package):
+    """Pmw is a toolkit for building high-level compound widgets, or megawidgets, constructed using other widgets as component parts."""
+    homepage = "https://pypi.python.org/pypi/Pmw"
+    url      = "https://pypi.python.org/packages/source/P/Pmw/Pmw-2.0.0.tar.gz"
+
+    version('2.0.0', 'c7c3f26c4f5abaa99807edefee578fc0')
+
+    extends('python')
+
+    def install(self, spec, prefix):
+        python('setup.py', 'install', '--prefix=%s' % prefix)