Namespace members lose their signatures in DTS snapshots

[jycouet]

Given:

export namespace ns {
  export function f(a: string): void
}

the DTS snapshot renders:

export declare namespace ns {
  export { f };
}

The signature of f is dropped (and nested namespaces lose everything below the first level), so changes to namespace member signatures don't invalidate the snapshot.

Context: trying tsnapi in sveltejs/cli (sveltejs/cli#1162), where sv-utils exposes most of its API through namespaces (js.imports.addDefault, ...) - none of those signatures end up guarded.