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-virtualenv/package.py

@@ -0,0 +1,16 @@
+from spack import *
+import shutil
+
+class PyVirtualenv(Package):
+    """virtualenv is a tool to create isolated Python environments."""
+    homepage = "http://virtualenv.readthedocs.org/projects/virtualenv/"
+    url      = "https://pypi.python.org/packages/source/v/virtualenv/virtualenv-1.11.6.tar.gz"
+
+    version('1.11.6', 'f61cdd983d2c4e6aeabb70b1060d6f49')
+    version('13.0.1', '1ffc011bde6667f0e37ecd976f4934db')
+
+    extends('python')
+    depends_on('py-setuptools')
+
+    def install(self, spec, prefix):
+        python('setup.py', 'install', '--prefix=%s' % prefix)