Make traits.examples.introduction a non-package?

After #1348, `traits.examples.introduction` is now a package that can be imported:
```
from traits.examples.introduction import *
```

This makes the `introduction` a package and makes it look like part of the public library. But I believe the folder of Python files is also intended as data, not package.

Note that `introduction` has an `__init__.py` file in the `introduction` folder. However I believe it was added before the `etsdemo` switched to using an `index.rst` file to capture the folder description (see https://github.com/enthought/traitsui/issues/849).