[Pubgrub] Lazily check for available versions
[Pubgrub] Lazily check for available versions

Otherwise, we end up doing a lot of unnecessary work upfront.

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