but I don't want to manually import all submodules.
There is no "I want" in Go. And there is no notion of "sub"-module, all modules are equal. As are all(*) packages. If you want to use a package you must import it. Dead simple.
Best advice: Stop making every package its own module; most likely this is overkill or plain wrong. Stop making tiny packages. Stop trying to mimick a source code layout you might be used from other languages. Read "How to Write Go Code" and stick to it word for word.
(*) internal and vendored packages are an exception which does not apply to your problem.