Fix sourcemap related warnings

```
WARNING in ../node_modules/react-jetpack/esm/utils/index.js
Module Warning (from ../node_modules/source-map-loader/dist/cjs.js):
Failed to parse source map from '/.../<dir>/node_modules/src/utils/index.ts' file: Error: ENOENT: no such file or directory, open '/.../<dir>/node_modules/src/utils/index.ts'
```