beets: Move into its own package directory.

The primary use of beets is not as a Python library and users usually
would expect to install it into the env using "nix-env -i beets" rather
than "nix-env -i pythonX.Y-beets".

Having beets in its own package directory also allows for better
customization, where we're going to implement attributes that can be
used to turn on/off various features and plugins.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>