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/hwloc/package.py

@@ -0,0 +1,25 @@
+from spack import *
+
+class Hwloc(Package):
+    """The Portable Hardware Locality (hwloc) software package
+       provides a portable abstraction (across OS, versions,
+       architectures, ...) of the hierarchical topology of modern
+       architectures, including NUMA memory nodes, sockets, shared
+       caches, cores and simultaneous multithreading. It also gathers
+       various system attributes such as cache and memory information
+       as well as the locality of I/O devices such as network
+       interfaces, InfiniBand HCAs or GPUs. It primarily aims at
+       helping applications with gathering information about modern
+       computing hardware so as to exploit it accordingly and
+       efficiently."""
+    homepage = "http://www.open-mpi.org/projects/hwloc/"
+    url      = "http://www.open-mpi.org/software/hwloc/v1.9/downloads/hwloc-1.9.tar.gz"
+
+    version('1.9', '1f9f9155682fe8946a97c08896109508')
+
+    def install(self, spec, prefix):
+        configure("--prefix=%s" % prefix)
+
+        make()
+        make("install")
+