PackageLoading: use `compactMap` instead of `flatMap` (NFC)
PackageLoading: use `compactMap` instead of `flatMap` (NFC)

This just replaces `flatpMap` which is deprecated in favour of
`compactMap` to avoid a deprecation warning.  NFCI.

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