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/glog/package.py

@@ -0,0 +1,15 @@
+import os
+from spack import *
+
+class Glog(Package):
+    """C++ implementation of the Google logging module."""
+
+    homepage = "https://github.com/google/glog"
+    url      = "https://github.com/google/glog/archive/v0.3.3.tar.gz"
+
+    version('0.3.3', 'c1f86af27bd9c73186730aa957607ed0')
+
+    def install(self, spec, prefix):
+        configure("--prefix=" + prefix)
+        make()
+        make("install")