New tutorials: write section to define and describe "package"

(For more information about this issue, please see #194).

We need a short section to follow the prerequisites section (see #196) (i.e., as its own file). It should satisfy the following goals:

• explain what a distribution package is
• explain how a distribution package differs from import packages and modules
• use the terminology that the wider Python Packaging User Guide uses, so the terminology is familiar if the reader turns to another part of the guide

I also think it would be helpful if the section could illustrate each point with some kind of example, either a made-up example or a real-world example, if it's not too complicated.

When opening pull requests for this issue, please submit to the develop branch. If you have questions that concern contributing more generally, please use issue #194. Otherwise, feel free to comment with questions or feedback. Thanks!

Inizia esaminando la sezione dei prerequisiti indicata nell’issue e le voci del glossario collegate della Python Packaging User Guide relative a distribution packages, import packages e modules. Il lavoro è completo quando un file di sezione autonomo segue i prerequisiti, utilizza la terminologia della guida e include esempi chiari che distinguono ciascun concetto; invia le modifiche al develop branch.