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/mrnet/package.py

@@ -0,0 +1,20 @@
+from spack import *
+
+class Mrnet(Package):
+    """The MRNet Multi-Cast Reduction Network."""
+    homepage = "http://paradyn.org/mrnet"
+    url      = "ftp://ftp.cs.wisc.edu/paradyn/mrnet/mrnet_4.0.0.tar.gz"
+
+    version('4.0.0', 'd00301c078cba57ef68613be32ceea2f')
+    version('4.1.0', '5a248298b395b329e2371bf25366115c')
+
+    parallel = False
+
+    depends_on("boost")
+
+    def install(self, spec, prefix):
+        configure("--prefix=%s" %prefix, "--enable-shared")
+
+        make()
+        make("install")
+