party-robot: teach package-import concept

[ErikSchierboom]

The party-robot expects students to use the fmt package, but its introduction.md document nor its dependencies explain how to import a package using the import keyword.

This exercise should explain how to import packages in the party-robot exercise. This means updating the introduction.md file, adding a concept to the concepts array and creating the files for that concept.

[bobtfish]

Hey @ErikSchierboom I see that there's already a packages concept that just needs some work, so I've started work on getting that into shape in PR#1651 (very much a WIP still right now!).

Do you have strong feelings about the name of the concept? I.e. packages which was pre-existing vs package-import as in this ticket. I have a mild preference for the former as think covering both declaring and importing packages at the same time makes sense, but I'm happy to take your advice.