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/cairo/package.py

@@ -0,0 +1,19 @@
+from spack import *
+
+class Cairo(Package):
+    """Cairo is a 2D graphics library with support for multiple output devices."""
+    homepage = "http://cairographics.org"
+    url      = "http://cairographics.org/releases/cairo-1.14.0.tar.xz"
+
+    version('1.14.0', 'fc3a5edeba703f906f2241b394f0cced')
+
+    depends_on("libpng")
+    depends_on("glib")
+    depends_on("pixman")
+    depends_on("fontconfig@2.10.91:") # Require newer version of fontconfig.
+
+    def install(self, spec, prefix):
+        configure("--prefix=%s" % prefix,
+                  "--enable-tee")
+        make()
+        make("install")