match resource should not include special .webpack[...] extension

[sokra]

.webpack[] should not be used for resource path

What kind of change does this PR introduce?
fixes webpack-contrib/mini-css-extract-plugin#755

Did you add tests for your changes?
yes

Does this PR introduce a breaking change?
no

What needs to be documented once your changes are merged?
nothing

[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.