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/pixman/package.py

@@ -0,0 +1,18 @@
+from spack import *
+
+class Pixman(Package):
+    """The Pixman package contains a library that provides low-level
+       pixel manipulation features such as image compositing and
+       trapezoid rasterization."""
+    homepage = "http://www.pixman.org"
+    url      = "http://cairographics.org/releases/pixman-0.32.6.tar.gz"
+
+    version('0.32.6', '3a30859719a41bd0f5cccffbfefdd4c2')
+
+    depends_on("libpng")
+
+    def install(self, spec, prefix):
+        configure("--prefix=%s" % prefix,
+                  "--disable-gtk")
+        make()
+        make("install")