Turbopack: don't treat metadata routes as RSC (#82911)
Metadata routes are just `route.ts`\-s in diguise.
Treat them the same regarding the layer (`app-route`) and whether to emit client reference/server actions manifests for them (= don't).
This fixes a panic:
```
thread 'tokio-runtime-worker' panicked at crates/next-api/src/dynamic_imports.rs:70:26:
called `Option::unwrap()` on a `None` value
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```
https://vercel.slack.com/archives/C05TH5T3DS8/p1755670936074369
