Describe the bug
When importing a library on a single page, it's bundled in the vendor-<hash>.js file and loaded on every page.
This has not been the case with Sapper.

To Reproduce

I've created two example projects, one with Sapper and one with Svelte-Kit.
They can be found here:

• https://github.com/Bastian/sapper-codesplitting (Relevant commit: Bastian/sapper-codesplitting@ae75f2c)
• https://github.com/Bastian/sveltekit-codesplitting (Relevant commit: Bastian/sveltekit-codesplitting@ec1da79)

On both projects, I've imported the moment.js library, as it has a large and noticeable bundle size (~20kB gzipped) but every other library would show the same result. The library is imported on the About page and nowhere else. I've recorded a video that shows the Chrome dev tools when loading the page and then navigating to the About page:

Here's how it looks like in Sapper where the automatic code-splitting is working:

And here's Svelte Kit where the complete bundle is loaded immediately:

You can clearly see that in Sapper the ~20kB gzipped bundle is only loaded when navigating to the About page whereas in Svelte-Kit it's loaded immediately (included in the vendor-<hash>.js file).

Expected behavior

Automatic code splitting should make sure that the library is only loaded on pages that import it.

Severity

Not severe but an inconvenience with a non-neglectable impact on performance.

Additional context

You can enforce code-splitting by using dynamic imports. While this is a good temporary solution to mitigate the impact of larger libraries, doing this for every single imported library would be a major inconvenience and result in a lot of unwanted boilerplate code.