Turbopack: content hash polyfill sourcemap file (#94548)
The sourcemap file of the polyfill chunk wasn't actually content hashed.
it just used the name of the corresponding JS chunk.
It was still emitted into `_next/static/immutable`. So it wasn't
actually immutable.
Now, it's content hashed (and this also removed the ad-hoc sourcemap
construction from app.rs and instead uses the usual pattern that all
other chunks use to reference their corresponding sourcemap chunks).
We didn't catch this because it only happens with
`nextConfig.productionBrowserSourceMaps: true` which isn't the default.
The test now covers this as well
