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/libevent/package.py

@@ -0,0 +1,30 @@
+from spack import *
+
+class Libevent(Package):
+    """The libevent API provides a mechanism to execute a callback function
+       when a specific event occurs on a file descriptor or after a timeout has been
+       reached. Furthermore, libevent also support callbacks due to signals or regular
+       timeouts.
+    """
+
+    homepage = "http://libevent.org"
+    url      = "https://github.com/downloads/libevent/libevent/libevent-2.0.21-stable.tar.gz"
+    list_url = "http://libevent.org/old-releases.html"
+
+    version('2.0.21', 'b2405cc9ebf264aa47ff615d9de527a2')
+    version('2.0.20', '94270cdee32c0cd0aa9f4ee6ede27e8e')
+    version('2.0.19', '91111579769f46055b0a438f5cc59572')
+    version('2.0.18', 'aa1ce9bc0dee7b8084f6855765f2c86a')
+    version('2.0.17', 'dad64aaaaff16b5fbec25160c06fee9a')
+    version('2.0.16', '899efcffccdb3d5111419df76e7dc8df')
+    version('2.0.15', '2643abe7ba242df15c08b2cc14ec8759')
+    version('2.0.14', 'cac0f379da35d3b98f83ac16fcfe1df4')
+    version('2.0.13', 'af786b4b3f790c9d3279792edf7867fc')
+    version('2.0.12', '42986228baf95e325778ed328a93e070')
+
+
+    def install(self, spec, prefix):
+        configure("--prefix=%s" % prefix)
+
+        make()
+        make("install")