fix: paths in postcss sourcemap sources array (Sourcemap ... points to missing source files)

[bfanger]

When using Vite's new devSourcemap you'll be getting warnings like:

Sourcemap for ".../src/routes/__layout.svelte" points to missing source files

Example SvelteKit project: https://github.com/bfanger/svelte-project-template

// svelte.config.js
/** @type {import('@sveltejs/kit').Config} */
export default {
  preprocess: preprocess({ sourceMap: true }),
  kit: {
    vite: { css: { devSourcemap: true } },
  },
};

When no to: is passed to postcss(), PostCSS will use NodeJS current working directory as fallback, which generates invalid paths.

This PR adds the to parameter, which allows PostCSS to generate the correct relative paths into the sourcemap sources array.

[floehopper]

I've started seeing these warnings having just upgraded from v1.0.0-next.298 -> next.310, but I don't have devSourcemap enabled. Will this also be fixed by these changes?

[bfanger]

@floehopper Dig into your local node_modules/svelte-preprocess
Edit both node_modules/svelte-preprocess/dist/transformers/globalStyle.js & postcss.js

and replace:

from: filename,

with

from: filename, 
to: filename,

And see if that fixes it, if not then there is something else also generating invalid paths

[floehopper]

@bfanger Yes, that fixes them - thanks! 👍

[Gin-Quin]

I'm having this issue for Stylus files. I fixed it by applying path.resolve to all sourcemap sources in transformers/stylus.ts. I created a PR for that.