Fix dynamic sitemap detection (#60356)
### What
Fix bad detection of dynamic route of sitemap metadata route, the swc
AST check should process when the text are detected. But prevuious if
there's text with `generateSitemap` such as comment but not the actual
export it will fail.
### How
Add both checks on metadata loader side and detection helper side
* Only call `generateSitemaps` helper when the export existed
* Fix the helper detection logic (major part of this PR)
Fixes #59698
Closes #60344
Closes NEXT-2007
