Revise Swift Preview package proposal (#1092)
Revise Swift Preview package proposal (#1092)

* Add detail about individual packages to the Preview Package proposal

* Add links to implementation

* Provide text for monolithic package alternative

apple/swift-evolution,natecook1000/swift-evolution,hartbit/swift-evolution,apple/swift-evolution,abertelrud/swift-evolution,hartbit/swift-evolution,abertelrud/swift-evolution,natecook1000/swift-evolution