Add --dry-run flag to mypy --install-types

[JakeSummers]

Feature

When adding mypy to existing projects, you often need to track down the missing typesheds.

My projects dependencies are managed with poetry. What I would like to do is something like this:

$ mypy --install-types --dry-run
types-google-cloud-ndb
types-response

Then I can run a poetry add types-response for each of the items listed.

[hauntsaninja]

Thanks, we should do #10600 (comment) which I believe meets your use case