Include .flow files in the distributable package

[gajus]

I'd like to able to import the Options type, e.g.

import type {
  Options
} from 'prepack/lib/options.js.flow'

This is to enable type-hinting in https://github.com/gajus/prepack-webpack-plugin.

The current build strips types, or at least I don't see a way to import them.

The way I distribute my packages is using flow-copy-source, e.g. gajus/prepack-webpack-plugin@69c3464