[PubgrubTests] Simplify dependency mocking API
[PubgrubTests] Simplify dependency mocking API

More consistent renaming and moving the unversioned package
dependencies into a convenience constructor. Makes it a little less
verbose to work with.

apple/swift-package-manager,abertelrud/swift-package-manager,apple/swift-package-manager,abertelrud/swift-package-manager,abertelrud/swift-package-manager,apple/swift-package-manager,apple/swift-package-manager,abertelrud/swift-package-manager,abertelrud/swift-package-manager,apple/swift-package-manager