How to deal with Backend dependencies in Plots v2

[BeastyBlacksmith]

Summarizing from #5143:

Backends might need additional dependencies. Since they are package extensions they can't directly have dependencies. Thus we have 3 Options:

1. Add them as normal Dependencies to PlotsBase
2. Have wrapper packages for each backend and add them to those
3. Require the user to load the dependencies as well as the backends