Turbopack Experimental: Fix source maps in HMR (#5852)
This constructs correct `sourceMappingURL`s for the code injected by the
hmr client.
It removes the `source_maps` module from entirely, as it's not even used
by standard `--turbo` and the `next-dev` binary.
Test Plan:
- Opened a Next.js app with `--turbo-experimental`
- Change a file, verify the page is updated with HMR
- Confirm the source map for the updated file is correctly loaded in
DevTools
Closes WEB-1482
