Partial caching, expose webpack-sources

[sokra]

What kind of change does this PR introduce?
feature

Did you add tests for your changes?
partially, existing tests

Does this PR introduce a breaking change?
no

What needs to be documented once your changes are merged?
Plugins should consume require("webpack").sources instead of require("webpack-sources") to avoid version conflicts for persistent caching

[webpack-bot]

For maintainers only:

• This needs to be documented (issue in webpack/webpack.js.org will be filed when merged)
• This needs to be backported to webpack 4 (issue will be created when merged)

[webpack-bot]

Thank you for your pull request! The most important CI builds succeeded, we’ll review the pull request soon.

[webpack-bot]

I've created an issue to document this in webpack/webpack.js.org.