New dependencies can break updates

[danepowell]

It's common for packages to add dependencies on new versions of PHP or PHP extensions that may not be available in a user environment.

This leads to a bad UX where a user has a working Symfony CLI, runs self-update, and suddenly the CLI is broken because of the missing dependency and they can't even roll back if it can't bootstrap.

One solution (a bit fiddly, admittedly) would be for the self-update command to have a requirements checker that parses an updated composer.json and ensures that it's compatible with the current environment before allowing an update to continue.