Entrypoints integrity workaround needed to allow for assets-webpack-plugin or higher

[weaverryan]

See #968

ztoben/assets-webpack-plugin#392 makes our integrity calculation no longer work, as the files have not been emitted by the time processOutput() is called.

The most obvious fix would be to - if possible - use the existing https://github.com/ztoben/assets-webpack-plugin#integrity

If it IS possible (I don't know if it exposes all the same info we do), then we would need to deprecate the current format of the entrypoints.json file in favor of this new one.