Port from Ungoliant: `package` command

[Uinelj]

The package command is in two steps:

• Create one folder per language, and move splits into these folders
• Compute sha256sums of each split, putting them in a sha256sum file.

The command name might be ambiguous, and we might have a way of doing all of this better.

[Uinelj]

Code is here: https://github.com/oscar-project/ungoliant/blob/v1.2.3/src/processing/package.rs