Make the lookup of the PackageDescription dylib fall back on the versioned location if needed (#3560)
Make the lookup of the PackageDescription dylib fall back on the versioned location if needed (#3560)

This makes the use of the Manifest API directory (the directory in which PackageDescription.dylib and its interface is located) more robust, so that it works with mismatching toolchains.  This is a little tricky to unit-test from inside the codebase, since it involves having an old toolchain compared with the expected one.  Existing unit tests make sure there are no regressions.
apple/swift-package-manager,abertelrud/swift-package-manager,abertelrud/swift-package-manager,apple/swift-package-manager,abertelrud/swift-package-manager,apple/swift-package-manager,abertelrud/swift-package-manager,apple/swift-package-manager,apple/swift-package-manager,abertelrud/swift-package-manager