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!

まず、issue で参照されている前提条件のセクションと、Python Packaging User Guide にある distribution packages、import packages、modules のリンクされた用語集エントリを確認します。前提条件の後に独立したセクションファイルが続き、ガイドの用語を使用し、それぞれの概念を区別する明確な例を含んでいれば、作業は完了です。変更を develop branch に提出してください。