That assumption is a bit wrong, or at least the conclusion
drawn are not suitable. The stdlib is part of the Go installation
and not a "real" module: Modules are used to track dependencies
and select versions of dependencies. Any single Go installation
needs _exactly_ the stdlib provided by the installation and you cannot
use a "different version" of the stdlib than the one distributed in
the installation of Go itself. That's why the stdlib is not a "module"
for your projects.
But the Go installation itself needs other code and that code is
tracked as normal dependencies via modules that's why you'll
find several go.mod in the source tree of Go.
The fact that stdlib packages have short import paths predates
modules by a decade, is convenient and nothing would be gained
It might even be misleading as you neither can replace or require
file of your project.
The stdlib is not an external dependency to your code.
V.