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-setuptools/package.py

@@ -0,0 +1,15 @@
+from spack import *
+
+class PySetuptools(Package):
+    """Easily download, build, install, upgrade, and uninstall Python packages."""
+    homepage = "https://pypi.python.org/pypi/setuptools"
+    url      = "https://pypi.python.org/packages/source/s/setuptools/setuptools-11.3.tar.gz"
+
+    version('11.3.1', '01f69212e019a2420c1693fb43593930')
+    version('16.0', '0ace0b96233516fc5f7c857d086aa3ad')
+    version('18.1', 'f72e87f34fbf07f299f6cb46256a0b06')
+
+    extends('python')
+
+    def install(self, spec, prefix):
+        python('setup.py', 'install', '--prefix=%s' % prefix)