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/libpng/package.py

@@ -0,0 +1,15 @@
+from spack import *
+
+class Libpng(Package):
+    """libpng graphics file format"""
+    homepage = "http://www.libpng.org/pub/png/libpng.html"
+    url      = "http://download.sourceforge.net/libpng/libpng-1.6.16.tar.gz"
+
+    version('1.6.16', '1a4ad377919ab15b54f6cb6a3ae2622d')
+    version('1.6.15', '829a256f3de9307731d4f52dc071916d')
+    version('1.6.14', '2101b3de1d5f348925990f9aa8405660')
+
+    def install(self, spec, prefix):
+        configure("--prefix=%s" % prefix)
+        make()
+        make("install")