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/protobuf/package.py

@@ -0,0 +1,16 @@
+import os
+from spack import *
+
+class Protobuf(Package):
+    """Google's data interchange format."""
+
+    homepage = "https://developers.google.com/protocol-buffers"
+    url      = "https://github.com/google/protobuf/releases/download/v2.5.0/protobuf-2.5.0.tar.bz2"
+
+    version('2.5.0', 'a72001a9067a4c2c4e0e836d0f92ece4')
+
+    def install(self, spec, prefix):
+        configure("--prefix=" + prefix)
+        make()
+        make("check")
+        make("install")