Add a discussion on distribution packages vs. import packages

[jeanas]

PR #1424 as well as this comment from @willingc make me think that it would be nice to have a discussion document on the difference between distribution packages and import packages. These terms are already explained in the glossary, but with few details and quite technical language.

This could cover: the terminology; the fact that distribution packages can have names with - and . in them, unlike import packages which (in 99.999% of cases) have Python identifiers as names; the fact that usually but not always, a distribution package contains a single import package with the same name (except - possibly replaced with _).

I'm going to take a stab at this.