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/atk/package.py

@@ -0,0 +1,18 @@
+from spack import *
+
+class Atk(Package):
+    """ATK provides the set of accessibility interfaces that are
+       implemented by other toolkits and applications. Using the ATK
+       interfaces, accessibility tools have full access to view and
+       control running applications."""
+    homepage = "https://developer.gnome.org/atk/"
+    url      = "http://ftp.gnome.org/pub/gnome/sources/atk/2.14/atk-2.14.0.tar.xz"
+
+    version('2.14.0', 'ecb7ca8469a5650581b1227d78051b8b')
+
+    depends_on("glib")
+
+    def install(self, spec, prefix):
+        configure("--prefix=%s" % prefix)
+        make()
+        make("install")