Sourcemaps are broken on Next.js + Webpack 4 when auto wrapping instrumentation is enabled

[lforst]

When combining our Next.js auto wrapping of data-fetchers and API routes with webpack 4 the resolving of sourcemaps breaks and errors get resolved to the proxy modules we create.

Steps to reproduce

1. Create Next.js 10 (has webpack 4 by default) or 11 app (need to set webpack5: false in Next.js config)
2. Create API route
3. Throw Error in API route
4. Look at stack trace in Sentry
5. Observe that it resolves to the proxy file we generate instead of to the source of the Error