Improve search paths for XCFrameworks

[neonichu]

When packaging a Swift dylib as an XCFramework, its modules will be at the top-level, so that directory needs to become part of search paths.

fixes #5723

[neonichu]

@swift-ci please smoke test

[hassila]

Just did a simple test:

Built a local SwiftPM and just tried building my sample project with this patch applied instead of the standard toolchain 'swift build' one and it worked:

hassila@ice ~/G/prototype-library-evolution-client (main)> swift build
Building for debugging...
/Users/hassila/GitHub/prototype-library-evolution-client/Sources/PrototypeLibraryEvolutionClient/PrototypeLibraryEvolutionClient.swift:2:8: error: no such module 'PrototypeLibraryEvolution'
import PrototypeLibraryEvolution
       ^
/Users/hassila/GitHub/prototype-library-evolution-client/Sources/PrototypeLibraryEvolutionClient/PrototypeLibraryEvolutionClient.swift:2:8: error: no such module 'PrototypeLibraryEvolution'
import PrototypeLibraryEvolution
       ^

hassila@ice ~/G/prototype-library-evolution-client (main) [1]> ../swift-package-manager/.build/debug/swift-build
Building for debugging...
[3/3] Linking PrototypeLibraryEvolutionClient
Build complete! (0.34s)
hassila@ice ~/G/prototype-library-evolution-client (main)> ls

[abertelrud]

Thanks for trying it out @hassila! @neonichu, this just needs tests, or is there more to the fix?

[neonichu]

@swift-ci please smoke test

[neonichu]

@swift-ci please smoke test macOS

[tonyarnold]

@neonichu will this fix be in the next public release of Swift? (after Swift 5.7.1)

[neonichu]

I think we haven't branched for 5.8 yet, so that should be the case.

[tonyarnold]

Thanks! I appreciate the info 😄