preprocess passing map has no effect

[shirotech]

Describe the bug
When I run preprocess, with passing {code, map, dependencies} it seems the map object has no effect, as the processed.css.map.sources has been lost and only contains the <component>.svelte file and no longer has *.scss files which were imported.

Expected behavior
It is expected to see in the sourcemap constructed uses the map object as a base which was passed rather than starting from scratch. e.g. the original .scss files with original @extend and @include sources, but what we only see are what was generated and it is hard to pinpoint the origin of the real source.

Information about your Svelte project:

• Your operating system: (e.x. OS X 10, Ubuntu Linux 19.10, Windows XP, etc)
  Mac

• Svelte version (Please check you can reproduce the issue with the latest release!)
  3.12.1

• Whether your project uses Webpack or Rollup
  Rollup

[Conduitry]

https://svelte.dev/docs#svelte_preprocess

This is a long-standing TODO. There's an old PR for it - #1863 - which might have some salvageable pieces.