Add more clarity to the packaging tutorial dir structure

[dogweather]

The tutorial begins by instructing the reader to create this confusing hierarchy without explanation:

example_pkg/
    example_pkg/
        __init__.py

The tutorial then continues on, referring to example_pkg frequently, but not clarifying which one is meant. Confusing! And not necessary, in my opinion. So this PR renames the top-level dir, better describing its purpose:

packaging_tutorial/
    example_pkg/
        __init__.py

[ncoghlan]

@theacodes This change seems like a good idea to me - the current structure came from the fact that in a typical single-package project the VCS repo and the top-level Python import package share a name (so you end up with this doubly-nested layout), but for someone running through the tutorial in a throwaway directory, that reasoning doesn't apply.

[theacodes]

Agreed. Thanks for doing this, @dogweather!