PackageDescription: fix access to underscored properties (#3548)
PackageDescription: fix access to underscored properties (#3548)

Underscored properties of `Target` shouldn't be available to callers of the PackageDescription API. The public access was probably a typo.
apple/swift-package-manager,abertelrud/swift-package-manager,abertelrud/swift-package-manager,abertelrud/swift-package-manager,apple/swift-package-manager,abertelrud/swift-package-manager,apple/swift-package-manager,apple/swift-package-manager,apple/swift-package-manager,abertelrud/swift-package-manager