improve warning for unresolved implicit dependencies

Rollup reports:

> […] is imported by […], but could not be resolved – treating it as an external dependency

https://github.com/rollup/rollup/blob/31e7348061597fd77f7154e327ec94acbe18fed2/src/utils/error.ts#L225

that can be confusing, so we might wanna provide more guidance (e.g. recommending `npm install`?)