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/tmux/package.py

@@ -0,0 +1,24 @@
+from spack import *
+
+class Tmux(Package):
+    """tmux is a terminal multiplexer. What is a terminal multiplexer? It lets
+       you switch easily between several programs in one terminal, detach them (they
+       keep running in the background) and reattach them to a different terminal. And
+       do a lot more.
+    """
+
+    homepage = "http://tmux.sourceforge.net"
+    url      = "http://downloads.sourceforge.net/project/tmux/tmux/tmux-1.9/tmux-1.9a.tar.gz"
+
+    version('1.9a', 'b07601711f96f1d260b390513b509a2d')
+
+    depends_on('libevent')
+    depends_on('ncurses')
+
+    def install(self, spec, prefix):
+        configure(
+            "--prefix=%s" % prefix,
+            "PKG_CONFIG_PATH=%s:%s" % (spec['libevent'].prefix, spec['ncurses'].prefix))
+
+        make()
+        make("install")