Hash doesn't change when using ModuleConcatenationPlugin

[adamesque]

We've noticed that when using this plugin with webpack 3 and ModuleConcatenationPlugin, the hash doesn't update when our source files change.

@qswitcher noticed that _source is empty for modules of type ConcatenatedModule, so (I believe) any module considered for scope hoisting will not have its contents included in the chunk hash. The source for these modules does seem to be present in module.rootModule._source, though, so it's possible you could use that for ConcatenatedModules.