Swift test uses the test frameworks out of the toolchain. You usually don’t need a package.
That said. I would then compare it to swift build where you specify a --build-system argument to select which build system to use. Mind you that’s an extreme since the build systems aren’t pluggable through packages (maybe they should be 
).
I think swift doc provides the best ergonomics for users. We can add options if they want to use a different documentation generator. And it would be good for documentation generator providers to be able to tag their generators in their package manifest so we can cleanly add them as options.
Anyway, lots more to consider here. Let’s make sure if we’re going to change things up that we take as much as we can think of into the solution.