For example, golang.org/x/tools should list the nested module golang.org/x/tools/gopls.
On second thought, I don't think we should do this. The documentation for a module is guaranteed to stay the same, which allows us to cache package documentation pages. Adding nested modules to the list of directories breaks this, as they might not be added to the database until later. Nested modules seem rare enough that this probably is not so important to implement.