Respect build.format = "preserve" option

[g7s]

Describe the bug
According to the Astro docs for build format, the "preserve" option will respect the directory/file structure. However the i18n:sync command does not and always generates index.astro pages.

To Reproduce
Steps to reproduce the behavior:

1. Set build.format to "preserve"
2. Run npm run i18n:sync and observe that's not taken into account

Expected behavior

To respect the build.format "preserve" option