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/pango/package.py

@@ -0,0 +1,19 @@
+from spack import *
+
+class Pango(Package):
+    """Pango is a library for laying out and rendering of text, with
+       an emphasis on internationalization. It can be used anywhere
+       that text layout is needed, though most of the work on Pango so
+       far has been done in the context of the GTK+ widget toolkit."""
+    homepage = "http://www.pango.org"
+    url      = "http://ftp.gnome.org/pub/gnome/sources/pango/1.36/pango-1.36.8.tar.xz"
+
+    version('1.36.8', '217a9a753006275215fa9fa127760ece')
+
+    depends_on("harfbuzz")
+    depends_on("cairo")
+
+    def install(self, spec, prefix):
+        configure("--prefix=%s" % prefix)
+        make()
+        make("install")