Linking Project Watchtower into projects with yarn link unfortunately does not work. Here's one (hacky) way to test unpublished versions:
Build and pack Project Watchtower into a .tgz file:
yarn build
npm pack
In the project you want to use it, add the .tgz file as a dependency:
yarn remove project-watchtower
yarn add file:/../project-watchtower-0.0.1.tgz
To update:
yarn cache clean
rm -rf node_modules
rm yarn.lock
yarn