Simplify module registry and import system

Sometimes, `System.import` returns an empty module, when it should return a module with all the exports (such as `setupOpenMRS`). This comes up when trying to develop patient chart widgets using import map overrides. This became a blocker for testing the app shell.

This happens because of the weird fusion between SystemJS and Webpack Module Federation that we are using for modules. We should just switch all the way to Webpack Module Federation.