Reproduction link or steps

https://stackblitz.com/edit/github-cuq1c6zc-syns2mmt?file=tsdown.config.ts

What is expected?

tsdown should externalize @types/ dependencies

What is actually happening?

tsdown is not externalizing @types/ dependencies when there isn't an actual package.

Any additional comments?

Found in flint-fyi/flint#2213 (where this is catching it correctly, but I couldn't fix it sufficiently)