build: list the `PackageLoading` dependency
build: list the `PackageLoading` dependency

`Sources/Workspace/UserToolchain.swift` references `PackageLoading` but
the build did not list the dependency.  Add the missing dependency.

abertelrud/swift-package-manager,apple/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