This was overlooked in #19458. Using the fully qualified name of each
module usually makes sense, but there is one module where it does not,
namely, the root module, since its name is `root`. The original Autodoc
tar creation logic used `comp.root_name` for the root module back when
it was the only module included in `sources.tar`, and that made sense.
Now, we get the best of both worlds, using the proper root name for the
root module while using the module name for the rest.
4fac5bd601