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/glib/package.py

@@ -0,0 +1,18 @@
+from spack import *
+
+class Glib(Package):
+    """The GLib package contains a low-level libraries useful for
+       providing data structure handling for C, portability wrappers
+       and interfaces for such runtime functionality as an event loop,
+       threads, dynamic loading and an object system."""
+    homepage = "https://developer.gnome.org/glib/"
+    url      = "http://ftp.gnome.org/pub/gnome/sources/glib/2.42/glib-2.42.1.tar.xz"
+
+    version('2.42.1', '89c4119e50e767d3532158605ee9121a')
+
+    depends_on("libffi")
+
+    def install(self, spec, prefix):
+        configure("--prefix=%s" % prefix)
+        make()
+        make("install")