Effectful imports should ignore sideEffects field

Do you want to request a feature or report a bug?

feature

What is the current behavior?

import 'list/fantasy-land'

with

sideEffects: false

does not include imported file in the bundle, which is somewhat OK

If the current behavior is a bug, please provide the steps to reproduce.

not a bug

What is the expected behavior?

this import is clearly effectful, so it actually IMHO should not get dropped regardless of the sideEffects

If this is a feature request, what is motivation or use case for changing the behavior?

described in expected behaviour

Please mention other relevant information such as the browser version, Node.js version, webpack version, and Operating System.

webpack@4, others - irrelevant

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