[flutter_plugin_tools] Don't require version changes for `dev_dependencies`

[stuartmorgan-g]

In flutter/plugins#5703 the publishable check thought there needed to be a version change, because pubspec.yaml changed. However, the only changes were to dev_dependencies, which don't need to be published. Ideally we should teach the tooling to know that's okay so it doesn't need overriding. (It would require much deeper analysis than that tooling is currently doing though, so it may not be worth the effort for an edge case.)

[stuartmorgan-g]

Now that we have the "is dev only" logic in the tooling, this is where we would want to put that logic (which would now exempt it from both version and changelog changes).