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/libxml2/package.py

@@ -0,0 +1,20 @@
+from spack import *
+
+class Libxml2(Package):
+    """Libxml2 is the XML C parser and toolkit developed for the Gnome
+       project (but usable outside of the Gnome platform), it is free
+       software available under the MIT License."""
+    homepage = "http://xmlsoft.org"
+    url      = "http://xmlsoft.org/sources/libxml2-2.9.2.tar.gz"
+
+    version('2.9.2', '9e6a9aca9d155737868b3dc5fd82f788')
+
+    extends('python')
+    depends_on('zlib')
+    depends_on('xz')
+
+    def install(self, spec, prefix):
+        configure("--prefix=%s" % prefix)
+
+        make()
+        make("install")