Builds with a Swift toolchain outside Xcode

A Swift toolchain installed beside Xcode contributes the compiler, but the linker and the software development kit still come from Xcode. Apple builds now resolve those separately and pass the linker's location to every compile and link, so a package that needs a newer compiler than the one Xcode ships builds without falling back to whatever the surrounding shell happened to expose.

Swift Testing follows the same rule. A macro plugin only matches the testing library from its own toolchain, and the two ship on different schedules, so test targets now compile and link against the copy the selected compiler provides when it has one, and against the version Xcode publishes otherwise. Test bundles that previously failed to expand @Test and @Suite build again.

Swift package libraries are also force-loaded into what depends on them, the way Swift Package Manager links every object file a target produced. A source file whose only contribution is a protocol conformance is no longer dropped, so conformances declared in an extension are found at runtime.