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/graphviz/package.py

@@ -0,0 +1,21 @@
+from spack import *
+
+class Graphviz(Package):
+    """Graph Visualization Software"""
+    homepage = "http://www.graphviz.org"
+    url      = "http://www.graphviz.org/pub/graphviz/stable/SOURCES/graphviz-2.38.0.tar.gz"
+
+    version('2.38.0', '5b6a829b2ac94efcd5fa3c223ed6d3ae')
+
+    parallel = False
+
+    depends_on("swig")
+    depends_on("python")
+    depends_on("ghostscript")
+
+    def install(self, spec, prefix):
+        configure("--prefix=%s" %prefix)
+
+        make()
+        make("install")
+