https://github.com/apple/swift-package-manager/pull/2253 swift-format は公式のToolchainには入っていないのでまだ PATH に自前インストールしておく必要があるんですね。 (edited)

[Commands] Add exprimental support for swift-format by aciidb0mb3r ...

This adds a swift package _format command to format all the sources in a package. The swift-format binary will be looked up in PATH and SWIFT_FORMAT. Arguments after -- will passed to swift-format ...