[DAEMON-318] Investigate upgrading to Yarn 2

Yarn 2 was released and we should investigate the upgrade process. Yarn is used for the top-level monorepo as well as appcd-default-plugins. We also need to check if we even need Lerna anymore. It may be unnecessary if there is no value and if we mandate Yarn over npm. Release notes: https://dev.to/arcanis/introducing-yarn-2-4eh1. The code in appcd-default-plugins will likely be moved into appcd-plugin where the plugin management lives, but the logic should be relatively unchanged. Also, Yarn 2 has an experimental plugin called "version" which assists with independent package versioning as is the case in the Appc Daemon's monorepo. For more info: https://next.yarnpkg.com/features/release-workflow.