webpack-core is obsolete for webpack 2; use webpack-sources to get RawSource

[sompylasar]

The latest version of this package as of now is declared compatible with webpack 2 only.

To reuse the webpack's RawSource class, ChunkManifestPlugin depends on a webpack-core package which is generally missing from a webpack 2 setup (this is easily seen in an npm-shrinkwrap.json diff after chunk-manifest-webpack-plugin is installed).

webpack-core was marked as obsolete for webpack 2, and instead webpack-sources should be used to reuse the various *Source from webpack.

This is how webpack 2 itself uses it:
https://github.com/webpack/webpack/blob/028c51301733836abbedc88be7483af2623f5943/lib/RawModule.js#L9

const RawSource = require("webpack-sources").RawSource;