pub.dev deducts points for having a pubspec.yaml `description` that is too short or too long; several of our plugins are losing points on this. To ensure that we are following—and modeling—best practices, this adds a check that our `description` fields meet pub.dev expectations.

Fixes our existing violations. Two are not published even though this only takes effect once published:
- camera: We change this plugin pretty frequently, so this should go out soon without adding a release just for this trivial issue.
- wifi_info_flutter: This is deprecated, so we don't plan to release it. It has to be fixed to allow the tool change to land though.