Refactor section functions, modules, libraries, packages

[bencomp]

Even to me it is confusing to talk about functions, classes, modules, libraries and packages. I don't think the level of detail in the explanations helps our learners use the concepts.

• We should have a look at the concepts we introduce and the order of their introduction, so that they build on each other.
• Add a warning that not all available packages are published in good faith and explain how you can reasonably understand which ones are safe to install
• Update comments in exercise 7.1 to make extra clear what isPass should return (string or boolean)