Per codesearch, the only place that the mediawiki.watchstar.widgets ResourceLoader module is used is as a run-time dependency of the mediawiki.page.watch.ajax module (loaded via mw.loader.load and mw.loader.using when needed if watchlist expiry is enabled). I propose that the widget be merged into the watch module, unless there are plans to use it elsewhere. The widget was originally introduced to be used in the watch module, rMW6224ffd03ccf: Replace toast with OOUI popup on watchlist message / T249259: Watchlist Expiry: Update popup loading behavior when watching via star [medium], and I couldn't find references to any other uses in the past.
This could cause a minor performance hit if additional loading is needed for the dependencies of the widget that aren't being loaded immediately for the overall watch button. If this is an issue, we can make is so that the dependencies of the widget still only get loaded when needed by putting them in the mw.loader.load / .using calls instead of the widget itself, and having the widget code not be run until the dependencies are loaded.