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/graphlib/package.py

@@ -0,0 +1,14 @@
+from spack import *
+
+class Graphlib(Package):
+    """Library to create, manipulate, and export graphs Graphlib."""
+    homepage = "http://https://github.com/lee218llnl/graphlib"
+    url      = "https://github.com/lee218llnl/graphlib/archive/v2.0.0.tar.gz"
+
+    version('2.0.0', '43c6df84f1d38ba5a5dce0ae19371a70')
+
+    def install(self, spec, prefix):
+        cmake(".", *std_cmake_args)
+
+        make()
+        make("install")