Cannot find anything about these warning in dub

[solidstate1991]

```
> dub upgrade
Upgrading project in L:\Users\laszl\Documents\GitHub\iota\
   Warning Sub package nulib:os_posix, referenced by nulib 
dub.package_.Package must be referenced using the path to its 
base package
   Warning Sub package nulib:os_win32, referenced by nulib 
dub.package_.Package must be referenced using the path to its 
base package
```

These in turn likely cause files contained within the subpackages 
to not be found by the compiler.