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/dri2proto/package.py

@@ -0,0 +1,14 @@
+from spack import *
+
+class Dri2proto(Package):
+    """DRI2 Protocol Headers."""
+    homepage = "http://http://cgit.freedesktop.org/xorg/proto/dri2proto/"
+    url      = "http://xorg.freedesktop.org/releases/individual/proto/dri2proto-2.8.tar.gz"
+
+    version('2.8', '19ea18f63d8ae8053c9fa84b60365b77')
+
+    def install(self, spec, prefix):
+        configure("--prefix=%s" % prefix)
+
+        make()
+        make("install")