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-pyqt/package.py

@@ -0,0 +1,24 @@
+from spack import *
+
+class PyPyqt(Package):
+    """PyQt is a set of Python v2 and v3 bindings for Digia's Qt
+       application framework and runs on all platforms supported by Qt
+       including Windows, MacOS/X and Linux."""
+    homepage = "http://www.riverbankcomputing.com/software/pyqt/intro"
+    url      = "http://sourceforge.net/projects/pyqt/files/PyQt4/PyQt-4.11.3/PyQt-x11-gpl-4.11.3.tar.gz"
+
+    version('4.11.3', '997c3e443165a89a559e0d96b061bf70')
+
+    extends('python')
+    depends_on('py-sip')
+
+    # TODO: allow qt5 when conditional deps are supported.
+    # TODO: Fix version matching so that @4 works like @:4
+    depends_on('qt@:4')
+
+    def install(self, spec, prefix):
+        python('configure.py',
+               '--confirm-license',
+               '--destdir=%s' % site_packages_dir)
+        make()
+        make('install')