Add fileName to the webpack loader params, allowing users to identify… (#634)

Co-authored-by: mattcompiles <[email protected]>

Author: phil-lgr

.changeset/nervous-beans-destroy.md

@@ -0,0 +1,7 @@
+---
+'@vanilla-extract/webpack-plugin': patch
+---
+
+Add fileName to the CSS virtual loader params
+
+This allows users to identify the source file for vanilla-extract css imports

packages/webpack-plugin/src/loader.ts

@@ -83,6 +83,7 @@ export function pitch(this: LoaderContext) {
         serializeVirtualCssPath: async ({ fileName, source }) => {
           const serializedCss = await serializeCss(source);
           const virtualResourceLoader = `${virtualLoader}?${JSON.stringify({
+            fileName: fileName,
             source: serializedCss,
           })}`;