Ref T13306. Currently, there's no easy way to import a third-party local-disk file dump into a Phacility instance.
Add some more options to bin/files migrate to support this. In particular, this enables:
$ ./bin/files --from-engine local-disk --engine amazon-s3 --local-disk-source path/to/backup
...to import these files into S3 directly.
These are general-purpose options and theoretically useful in other use cases, although realistically those cases are probably very rare.