Refactor dist init and split out dist migrate

[duckinator]

• dist init still works as on main
• dist migrate does the migration part of dist init
• dist init's migration functionality is now implemented in terms of dist migrate

This shows dist migrate on its own:

And this shows that dist migrate is only a migration, and explicitly does not do the re-init part:

[mistydemeo]

Looking good. Tested that dist init on a new, uninitted workspace works fine, and that dist migrate also works on an initted workspace with Cargo.toml. Likewise, dist init with or without the migration is doing the right thing.