sass-loader: Resolve isn't looking at main fields like _index.scss

[peterhass]

Since I use storybook and rollup in the same package, I noticed some differences in how module resolve during import works.

The package I am trying to import holds a file called _index.scss. node_modules/my-ui-kit/_index.scss
Inside some scss file I am trying to build, I try to import it like I would with webpack's sass-loader:

@import "~my-ui-kit";

webpack's sass-loader would load node_modules/my-ui-kit/_index.scss but rollup-plugin-postcss just fails. It falsely assumes that my-ui-kit is the partial and tries to load /_my-ui-kit/.

https://github.com/egoist/rollup-plugin-postcss/blob/master/src/sass-loader.js#L63
partialUrl = "/_my-ui-kit"

webpack sass-loader passes mainFields: ['_index', 'index'] to resolve - maybe we could use something like this too? https://github.com/webpack-contrib/sass-loader/blob/master/src/utils.js#L351

Would be open to do a PR if you could give me some initial support. (Update: PR)