fix(admin-vite-plugin): Normalize file paths and add tests (#9595)

**What**
- #9338 had a regression which caused the import path in some virtual modules to be invalid on Windows.
- This PR fixes the issue so we now again create the correct import paths, and adds tests to prevent this from slipping in again.

packages/admin/admin-vite-plugin/src/utils.ts

@@ -17,7 +17,7 @@ import {
 } from "./babel"
 
 export function normalizePath(file: string) {
-  return path.normalize(file).split(path.sep).join("/")
+  return path.normalize(file.replace(/\\/g, "/"))
 }
 
 /**
@@ -145,3 +145,11 @@ export function isFileInAdminSubdirectory(
   const normalizedPath = normalizePath(file)
   return normalizedPath.includes(`/src/admin/${subdirectory}/`)
 }
+
+/**
+ * Test util to normalize strings, so they can be compared without taking
+ * whitespace into account.
+ */
+export function normalizeString(str: string): string {
+  return str.replace(/\s+/g, " ").trim()
+}