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/gdk-pixbuf/package.py

@@ -0,0 +1,22 @@
+from spack import *
+
+class GdkPixbuf(Package):
+    """The Gdk Pixbuf is a toolkit for image loading and pixel buffer
+       manipulation. It is used by GTK+ 2 and GTK+ 3 to load and
+       manipulate images. In the past it was distributed as part of
+       GTK+ 2 but it was split off into a separate package in
+       preparation for the change to GTK+ 3."""
+    homepage = "https://developer.gnome.org/gdk-pixbuf/"
+    url      = "http://ftp.gnome.org/pub/gnome/sources/gdk-pixbuf/2.31/gdk-pixbuf-2.31.1.tar.xz"
+
+    version('2.31.2', '6be6bbc4f356d4b79ab4226860ab8523')
+
+    depends_on("glib")
+    depends_on("jpeg")
+    depends_on("libpng")
+    depends_on("libtiff")
+
+    def install(self, spec, prefix):
+        configure("--prefix=%s" % prefix)
+        make()
+        make("install")