Handle multiple unused submodule imports

pylint does this properly, Flake8 does not.

For example, with this:

```py
import multiprocessing.pool
import multiprocessing.process
```

Flake8 will only mark the second import as unused.

Reproduce the reported example with the current linter and inspect how unused submodule imports are diagnosed. Compare the result with the expected behavior described for pylint; done means both unused imports are handled consistently rather than only the second one.

Written by the indexing model from the issue text.