fix: workaround for css-loader path issue

Since the css-loader changed in the current version to
require tilde as prefix for module imports and every other
import is taken as relative import, hops follows this
and deprecates unprefixed imports.
This in turn conflicts with the postcss-import handler
which does not like the tilde prefix and we remove it
again here, if its encountered.

packages/postcss/mixin.core.js

@@ -1,4 +1,5 @@
 const postcssImportPlugin = require('postcss-import');
+const postcssImportResolver = require('postcss-import/lib/resolve-id');
 const postcssPresetEnv = require('postcss-preset-env');
 const ExtractCSSPlugin = require('mini-css-extract-plugin');
 const FilterWarningsPlugin = require('webpack-filter-warnings-plugin');
@@ -37,7 +38,14 @@ const getCSSLoaderConfig = (browsers, additionalLoader) => {
         options: {
           ident: 'postcss',
           plugins: [
-            postcssImportPlugin(),
+            postcssImportPlugin({
+              resolve: (id, base, options) =>
+                postcssImportResolver(
+                  id.startsWith('~') ? id.slice(1) : id,
+                  base,
+                  options
+                ),
+            }),
             postcssPresetEnv({
               browsers,
               autoprefixer: {