[Expo] Prevent the use of react-native internals with react-native-web

Created by: EvanBacon

It's currently difficult to create more advanced aliases on top of react-native-web. We do the same in the expo-cli here. It'd be nice to emulate this without directly switching off of react-scripts. Here is an example that would work given this PR is merged: Code sandbox. You could then use Unimodules in code sandbox, or with react-scripts with minimal friction.

It's also bad practice to use react-native internals. The majority (if not all) of them don't work because the internal structure of react-native-web is optimized for babel-plugin-react-native-web.

CC: @necolas