Add a require_once to new classes BackupMigrateFilesDestinationConfigSource and BackupMigrateFilesDestinationDbConfigSource

[argiepiano]

The error reported in #158 may be related to the fact that new classes listed on the title of this issue do not use require_once to load their parent classes' files. Since BAM runs on cron, there may be issues with hook_autoload_info() not running, which may explain the errors seen there.

To keep the same pattern used in the rest of the BAM classes, we should instead explicitly load the parent class' file.

[argiepiano]

This is being blocked by #161. As soon as that is merged, I can provide a PR here.