fix: fallback page should use path not file location (#1301)

* 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).

* fix: use router.getFile to resolve fallback url

* specs: add cypress test for fallbackLanguages

Co-authored-by: 沈唁 <52o@qq52o.cn>