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/tsconfig.json

@@ -18,5 +18,5 @@
       "@babel/types": ["../../../node_modules/@babel/types"]
     }
   },
-  "include": ["src"]
+  "include": ["src", "vitest.config.ts", "tsup.config.ts"]
 }