[flutter_tools] Add plumbing for widget cache (#61766)

To support #61407 , the tool needs to check if a single widget reload is feasible, and then conditionally perform a fast reassemble.

To accomplish this, the FlutterDevice class will have a WidgetCache injected. This will eventually contain the logic for parsing the invalidated dart script. Concurrent with the devFS update, the widget cache will be updated/checked if a single widget reload is feasible. If so, an expression evaluation with the target type is performed and the success is communicated through the devFS result. An integration test which demonstrates that this works is already present in https://github.com/flutter/flutter/blob/master/packages/flutter_tools/test/integration.shard/hot_reload_test.dart#L86

Finally, when actually performing the reassemble the tool simply checks if this flag has been set and calls the alternative reassemble method.

Cleanups:

Remove modules, as this is unused now.