fix: always use / when splitting filename to get name and namespace (#5038)

Co-authored-by: Nolan Lawson <[email protected]>

Author: danjunger

packages/@lwc/rollup-plugin/src/index.ts

@@ -328,7 +328,9 @@ export default function lwc(pluginOptions: RollupLwcOptions = {}): Plugin {
             // Specifier will only exist for modules with alias paths.
             // Otherwise, use the file directory structure to resolve namespace and name.
             const [namespace, name] =
-                specifier?.split('/') ?? path.dirname(filename).split(path.sep).slice(-2);
+                // Note we do not need to use path.sep here because this filename contains
+                // a '/' regardless of Windows vs Unix, since it comes from the Rollup `id`
+                specifier?.split('/') ?? path.dirname(filename).split('/').slice(-2);
 
             const apiVersionToUse = getAPIVersionFromNumber(apiVersion);