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/libxcb/package.py

@@ -0,0 +1,21 @@
+from spack import *
+
+class Libxcb(Package):
+    """The X protocol C-language Binding (XCB) is a replacement 
+    for Xlib featuring a small footprint, latency hiding, direct 
+    access to the protocol, improved threading support, and 
+    extensibility."""
+
+    homepage = "http://xcb.freedesktop.org/"
+    url      = "http://xcb.freedesktop.org/dist/libxcb-1.11.tar.gz"
+
+    version('1.11', '1698dd837d7e6e94d029dbe8b3a82deb')
+    version('1.11.1', '118623c15a96b08622603a71d8789bf3')
+    depends_on("python")
+    depends_on("xcb-proto")
+
+    def install(self, spec, prefix):
+        configure("--prefix=%s" % prefix)
+
+        make()
+        make("install")