ToolsSupportCore の件ですが、Package.swift は

let package = Package( name: ... dependencies: [ .package(url: "https://github.com/apple/swift-tools-support-core", .branch("master")) ], targets: [ .target( name: "...", dependencies: ["SwiftToolsSupport"] ),

で、 ArgumentParser を使いたいとこで import TSCUtility でいけました

(edited)