Add `--ignore-overrides` option

[mosuem]

When using monorepos with Dart packages referencing each other, usually every package has a pubspec_overrides.yaml declared, overriding the dependency to the published version with a path one.

In the case where a developer makes changes to multiple packages and publishes only some of them, the tests might still be passing as they respect the pubspec_overrides, even if the published versions will fail to work.

Ideally, the CI should include two runs - one using the pubspec_overrides and one using the published deps. This is not supported at the moment, and an option to specify this would be a cleaner thing to do then to run

dart test
find -type f -name 'pubspec_overrides.yaml' -delete
dart test