fix: fallback page should use path not file location

When resolving fallback page for the path (`fallbackLanguages` option), we should consider the original path, not the file location (which could be different when aliases are used).

src/core/fetch/index.js

@@ -123,7 +123,7 @@ export function fetchMixin(proto) {
           this._loadSideAndNav(path, qs, loadSidebar, cb)
         ),
       _ => {
-        this._fetchFallbackPage(file, qs, cb) || this._fetch404(file, qs, cb);
+        this._fetchFallbackPage(path, qs, cb) || this._fetch404(file, qs, cb);
       }
     );