Feature: Fail on inclusion of "dangerous packages"

**What I'd like:** When troubleshooting an issue, I'll create an AMI which includes the `login` package in order to poke around the underlying host. One of my concerns is that I might include this package in my `Config.toml` by mistake and push it to a production environment.

I'd like to see some kind of mechanism in which "dangerous packages" can be added to a list. Should I build an AMI with this package included, then the build should fail. As a developer who needs to use these packages in a development environment, then I can include a flag to override this behavior (e.g. `--allow-unsafe-packages`).

This would open the door for introducing other packages which may never be included in prod (e.g. with `curl` or other helpful tools), but would be super helpful for debugging.

**Any alternatives you've considered:** Careful code reviews.