The one drawback to this change is that `theano.scan`, i.e. the `scan` function
exposed by `theano/__init__.py`, shadows the `theano.scan` sub-package.

In other words, modules in the `theano.scan` sub-package cannot be accessed via
the top-level `theano` module object (e.g. `import theano; theano.scan.op.Scan`
won't work).

This is a minor inconvenience, and, since internal library code is generally
expected to import objects directly from the modules in which they're defined,
the appearance of this problem will serve as a welcome warning.