autoupdate is not updating versions of libraries in additional_dependencies.

For example, I have this config:

-   repo: https://gitlab.com/pycqa/flake8
    rev: 3.7.8
    hooks:
    -   id: flake8
        additional_dependencies:
          - flake8-bugbear==18.8.0
          - flake8-builtins==1.4.1
          - flake8-comprehensions==1.4.1
          - flake8-tidy-imports==1.1.0

After running pre-commit autoupdate only the version of flake8 is updated to 3.7.9.

Two reasons to do update additional dependencies:

• old versions of libraries in additional dependencies may not be compatible with latest versions of top level hooks.
• in some cases these additional dependencies need to be in sync in multiple hooks, for example, yesqa needs the same dependencies as flake8.

I would be great if these dependencies were updated when running autoupdate.