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/libcircle/package.py

@@ -0,0 +1,18 @@
+import os
+from spack import *
+
+class Libcircle(Package):
+    """libcircle provides an efficient distributed queue on a cluster,
+       using self-stabilizing work stealing."""
+
+    homepage = "https://github.com/hpc/libcircle"
+
+    version('0.2.1-rc.1', '2b1369a5736457239f908abf88143ec2',
+             url='https://github.com/hpc/libcircle/releases/download/0.2.1-rc.1/libcircle-0.2.1-rc.1.tar.gz')
+
+    depends_on('mpi')
+
+    def install(self, spec, prefix):
+        configure("--prefix=" + prefix)
+        make()
+        make("install")