set `package_dir` more generally

Setting `package_dir` to `{ '': 'lib' }` makes `python setup.py
develop` (and, by extension, `pip install -e <checkout>`) work by
saying that *all* packages live under `lib` rather than specifically
the `ansible` package.

It does not address `library`, however.  Setting `ANSIBLE_LIBRARY`
is still required for that.

Tested under OS X and CentOS 6 in the `pip` environment.  `sdist`
output also appears correct.