Turbopack: fix subpath imports pointing to external packages (#93308)
Closes https://github.com/vercel/next.js/issues/93295
Closes #93914
https://nodejs.org/api/packages.html#subpath-exports:
> All target paths in the
["exports"](https://nodejs.org/api/packages.html#exports) map (the
values associated with export keys) must be relative URL strings
starting with `./`
but
>Unlike the "exports" field, the "imports" field permits mapping to
external packages.
>The resolution rules for the imports field are otherwise analogous to
the exports field.