Manage `ignore_missing_imports = true` sections configs automatically

Feature

When I'm adding typing to an existing project, I encounter many libraries that don't have type hints. The easy solution to suppressing these errors is setting ignore_missing_imports = true, but this is not great. The recommended solution, assuming you don't want to write your own stubs, is to create separate sections for each such library. This can be cumbersome to create and manage.

I wrote myself a small tool that generates such configs: https://github.com/indigoviolet/mypy-missing-imports . I think it would be nice for mypy to include something like it, to make the "recommended" path easier to follow.

Pitch

I'm imagining something like

mypy --generate-missing-import-configs <optionally, /path/to/mypy.ini>

which would print out sections for each library identified as a [import] error like

[mypy-foo.*]
ignore_missing_imports = true

If passed a configuration file, it would update the corresponding sections in that file.

I'd be happy to try and add this to mypy if it is useful.

Start with mypy's command-line handling and configuration processing, using the proposed mypy --generate-missing-import-configs command and optional mypy.ini input as entry points. Review the referenced mypy-missing-imports tool for the intended behavior. Done means generating or updating per-library sections such as [mypy-foo.*] with ignore_missing_imports = true.