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/docbook-xml/package.py

@@ -0,0 +1,19 @@
+import os
+import glob
+from spack import *
+
+
+class DocbookXml(Package):
+    """Docbook DTD XML files."""
+    homepage = "http://www.oasis-open.org/docbook"
+    url = "http://www.oasis-open.org/docbook/xml/4.5/docbook-xml-4.5.zip"
+
+    version('4.5', '03083e288e87a7e829e437358da7ef9e')
+
+    def install(self, spec, prefix):
+        cp = which('cp')
+
+        install_args = ['-a', '-t', prefix]
+        install_args.extend(glob.glob('*'))
+
+        cp(*install_args)