Verify that source maps are generated correctly

[AlexITC]

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While migrating to scalajs-bundler, I have changed code that touches the source maps generation (1e2e6fdaa302396f53ce89d55452b131d58ddd9c), I think that the approach changes from sbt-js-dependencies but I haven't put much time into verifying that it still works.

Let's verify that, and, if necessary, fix it.