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/snappy/package.py

@@ -0,0 +1,15 @@
+import os
+from spack import *
+
+class Snappy(Package):
+    """A fast compressor/decompressor: https://code.google.com/p/snappy"""
+
+    homepage = "https://code.google.com/p/snappy"
+    url      = "https://github.com/google/snappy/releases/download/1.1.3/snappy-1.1.3.tar.gz"
+
+    version('1.1.3', '7358c82f133dc77798e4c2062a749b73')
+
+    def install(self, spec, prefix):
+        configure("--prefix=" + prefix)
+        make()
+        make("install")