Turbopack: handle invalid RSC imports via importmap (#88146)
We have two lines of defense:
1. crates/next-custom-transforms/src/transforms/react_server_components.rs validates ESM imports of server-only and client-only and throws an issue if something is wrong. Notably, `require('server-only')` isn't caught by this
2. additionally, we had an `BeforeResolvePlugin` for server-only, client-only and styled-jsx.
We should probably get rid of the first one completely for consistency (it's brittle)
https://github.com/vercel/next.js/blob/79040a318d29641eb3d96a5a47c92a5f8506e826/crates/next-custom-transforms/src/transforms/react_server_components.rs#L663-L674
This PR is about improving the second one:
The error message isn't ideal yet, but better than before (note: no import trace):
```
./bench/basic-app/app
Invalid import
'server-only' cannot be imported from a Client Component module. It should only be used from a Server Component.
The error was caused by importing 'bench/basic-app/app'
```
now
<img width="708" height="386" alt="Bildschirmfoto 2026-01-07 um 17 21 59" src="https://github.com/user-attachments/assets/4d49eff3-4959-4b70-aa47-6f3c10c75588" />
