WatchMissingNodeModulesPlugin now handles webpack@4 data

Created by: GoodForOneFare

webpack now uses Sets to track:

• missingDependencies
• contextDependencies

Set#some does not exist, so this fix just converts webpack's data to an array. Set#push does not exist, so watch logic now uses add if it's available

---

To verify

Check out this example repo that runs a webpack@4 build with WatchMissingNodeModules as a plugin.

Before	After

Verify that react-dev-utils@next fails with a confusing error

git clone git@github.com:GoodForOneFare/watch-missing-modules-webpack4-example.git
cd watch-missing-modules-webpack4-example
git checkout master
yarn install
yarn run build # This will output errors about missingDeps.some not being a function

Verify that this fork gives useful feedback + watch behaviour

git clone git@github.com:GoodForOneFare/watch-missing-modules-webpack4-example.git
cd watch-missing-modules-webpack4-example
git checkout fixed
yarn install
yarn run build --watch # This will allow the plugin to report a missing dependency (the build still fails, though!)

# To verify that contextDependencies watching still works, use this:
#  `touch node_modules/missing`
# Once that folder exists, the build should pass.