yarn watch can unzip invalid backend package concurrently being built

`yarn watch` launches a number of concurrent commands via `build/yarn-scripts.ts`. Two are interacting:
- `yarn scalawatch`, which builds the backend package zip; and
- `webpack --watch --devtool nosources-source-map --config ./webpack/ext-dev.webpack.config.js`, which has a hook that unzips the backend package zip.

The first partially writes the zip when (potentially) the second unzips it (and fails). This then leaves the zip in an invalid state as both processes are killed when an error occurs.

Suggestion: perhaps a successful build of the backend package zip should copy it to a different directory (`dist`?), and the webpack unzip code should look for it there. That would make the zip build "atomic".

cc: @shanedell