typeshed's VERSIONS file now supports submodules

typeshed's `VERSIONS` file now supports sub-modules. For example:

```
importlib: 2.7-
importlib.resources: 3.7-
```

Currently, mypy just ignores sub-modules and treats them like their parent module. It would be useful if mypy would recognize that a sub-module was not available in a certain Python version, though.
