[Pitch] SwiftPM build configuration conditionals for target dependencies

Hello Swift community, This pitch re-proposes the unimplemented part of SE-0273: Package Manager Conditional Target Dependencies. The original proposal, accepted in 2019, intended to introduce two conditional mechanisms for target dependencies: platform conditionals and build configuration conditionals. While platform conditionals shipped in Swift 5.3, configuration conditionals were left unimplemented because SwiftPM still had to generate Xcode projects via swift package generate-xcodeproj, an...