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/xcb-proto/package.py

@@ -0,0 +1,15 @@
+from spack import *
+
+class XcbProto(Package):
+    """Protocol for libxcb"""
+
+    homepage = "http://xcb.freedesktop.org/"
+    url      = "http://xcb.freedesktop.org/dist/xcb-proto-1.11.tar.gz"
+
+    version('1.11', 'c8c6cb72c84f58270f4db1f39607f66a')
+
+    def install(self, spec, prefix):
+        configure("--prefix=%s" % prefix)
+
+        make()
+        make("install")