Previously, the Watcher was a separate package. It's currently consumed by:
* our integration tests
* our UI
* synthetix’s tests and UI

Given core-utils is a package meant to ship Optimism related utilities
to consumers, it makes more sense to bundle the Watcher in core-utils
than have it as a separate package.
Co-authored-by: Kelvin Fichter <kelvinfichter@gmail.com>