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/libdrm/package.py

@@ -0,0 +1,18 @@
+from spack import *
+
+class Libdrm(Package):
+    """A userspace  library for  accessing the  DRM, direct
+    rendering  manager, on  Linux,  BSD and  other  operating 
+    systems that support the  ioctl interface."""
+
+    homepage = "http://dri.freedesktop.org/libdrm/" # no real website...
+    url      = "http://dri.freedesktop.org/libdrm/libdrm-2.4.59.tar.gz"
+
+    version('2.4.59', '105ac7af1afcd742d402ca7b4eb168b6')
+    version('2.4.33', '86e4e3debe7087d5404461e0032231c8')
+
+    def install(self, spec, prefix):
+        configure("--prefix=%s" % prefix)
+
+        make()
+        make("install")