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/tk/package.py

@@ -0,0 +1,22 @@
+from spack import *
+
+class Tk(Package):
+    """Tk is a graphical user interface toolkit that takes developing
+       desktop applications to a higher level than conventional
+       approaches. Tk is the standard GUI not only for Tcl, but for
+       many other dynamic languages, and can produce rich, native
+       applications that run unchanged across Windows, Mac OS X, Linux
+       and more."""
+    homepage = "http://www.tcl.tk"
+    url      = "http://prdownloads.sourceforge.net/tcl/tk8.6.3-src.tar.gz"
+
+    version('src', '85ca4dbf4dcc19777fd456f6ee5d0221')
+
+    depends_on("tcl")
+
+    def install(self, spec, prefix):
+        with working_dir('unix'):
+            configure("--prefix=%s" % prefix,
+                      "--with-tcl=%s" % spec['tcl'].prefix.lib)
+            make()
+            make("install")