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/wx/package.py

@@ -0,0 +1,24 @@
+from spack import *
+
+class Wx(Package):
+    """wxWidgets is a C++ library that lets developers create
+       applications for Windows, Mac OS X, Linux and other platforms
+       with a single code base. It has popular language bindings for
+       Python, Perl, Ruby and many other languages, and unlike other
+       cross-platform toolkits, wxWidgets gives applications a truly
+       native look and feel because it uses the platform's native API
+       rather than emulating the GUI. It's also extensive, free,
+       open-source and mature."""
+    homepage = "http://www.wxwidgets.org/"
+
+    version('2.8.12', '2fa39da14bc06ea86fe902579fedc5b1',
+            url="https://sourceforge.net/projects/wxwindows/files/2.8.12/wxWidgets-2.8.12.tar.gz")
+    version('3.0.1', 'dad1f1cd9d4c370cbc22700dc492da31',
+            url="https://sourceforge.net/projects/wxwindows/files/3.0.1/wxWidgets-3.0.1.tar.bz2")
+
+    def install(self, spec, prefix):
+        configure("--prefix=%s" % prefix, "--enable-unicode", "--disable-precomp-headers")
+
+        make(parallel=False)
+        make("install")
+