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/libjson-c/package.py

@@ -0,0 +1,14 @@
+from spack import *
+
+class LibjsonC(Package):
+    """ A JSON implementation in C """
+    homepage = "https://github.com/json-c/json-c/wiki"
+    url      = "https://s3.amazonaws.com/json-c_releases/releases/json-c-0.11.tar.gz"
+
+    version('0.11', 'aa02367d2f7a830bf1e3376f77881e98')
+
+    def install(self, spec, prefix):
+        configure('--prefix=%s' % prefix)
+
+        make()
+        make("install")