Doesn't like dynamic imports

Plugin is fine when using without dynamic `import`s in Webpack – using `chunkFilename`. However, as soon as you try to build it with dynamic `import`s I get:

```
ERROR in ./node_modules/pdfkit/js/document.js
Module build failed: Error: /node_modules/pdfkit/js/document.js: Could not statically evaluate how the fs module was required/imported.
```

Start by reproducing the dynamic import build in Webpack and inspect the failure reported for node_modules/pdfkit/js/document.js. Trace how the plugin handles fs imports in dynamically loaded modules. Done means the build completes without the static-evaluation error while preserving the existing non-dynamic import behavior.

Written by the indexing model from the issue text.