MDL-37761 Backup: Do not include files in backups when importing courses/activities

When we import courses or duplicate activities, the deduplicating nature of
the file_storage system means that we don't actually have to include the
real files in the backup - just their metadata from the file table.

The restoration process needs to know not to expect files from the backup
phase so a flag is set in moodle_backup metadata.
MDL-37761 Backup: Add support for fileless SAMESITE backups
MDL-37761 Backup: Add phpdoc for MODE constants