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-pygments/package.py

@@ -0,0 +1,15 @@
+from spack import *
+
+class PyPygments(Package):
+    """Pygments is a syntax highlighting package written in Python."""
+    homepage = "https://pypi.python.org/pypi/pygments"
+    url      = "https://pypi.python.org/packages/source/P/Pygments/Pygments-2.0.1.tar.gz"
+
+    version('2.0.1', 'e0daf4c14a4fe5b630da765904de4d6c')
+    version('2.0.2', '238587a1370d62405edabd0794b3ec4a')
+
+    extends('python')
+    depends_on('py-setuptools')
+
+    def install(self, spec, prefix):
+        python('setup.py', 'install', '--prefix=%s' % prefix)