Distinguish between dependencies and extra packages in `project.toml`

[timueh]

This issue can be tackled as soon as there is a new version of Pkg.jl: https://github.com/JuliaLang/Pkg.jl/issues/876

As we need certain packages only for testing, we should specify this in project.toml, see the documentation of Pkg.

The project.toml was created like this, see commit 97f3af6, but that broke the build, see this post.