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-sip/package.py

@@ -0,0 +1,21 @@
+from spack import *
+import os
+
+class PySip(Package):
+    """SIP is a tool that makes it very easy to create Python bindings for C and C++ libraries."""
+    homepage = "http://www.riverbankcomputing.com/software/sip/intro"
+    url      = "http://sourceforge.net/projects/pyqt/files/sip/sip-4.16.5/sip-4.16.5.tar.gz"
+
+    version('4.16.5', '6d01ea966a53e4c7ae5c5e48c40e49e5')
+    version('4.16.7', '32abc003980599d33ffd789734de4c36')
+
+    extends('python')
+
+    def install(self, spec, prefix):
+        python('configure.py',
+               '--destdir=%s' % site_packages_dir,
+               '--bindir=%s' % spec.prefix.bin,
+               '--incdir=%s' % python_include_dir,
+               '--sipdir=%s' % os.path.join(spec.prefix.share, 'sip'))
+        make()
+        make('install')