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/libxshmfence/package.py

@@ -0,0 +1,16 @@
+from spack import *
+
+class Libxshmfence(Package):
+    """This is a tiny library that exposes a event API on top of Linux
+    futexes."""
+
+    homepage = "http://keithp.com/blogs/dri3_extension/" # not really...
+    url      = "http://xorg.freedesktop.org/archive/individual/lib/libxshmfence-1.2.tar.gz"
+
+    version('1.2', 'f0b30c0fc568b22ec524859ee28556f1')
+
+    def install(self, spec, prefix):
+        configure("--prefix=%s" % prefix)
+
+        make()
+        make("install")