[Accepted] SE-0273: Package Manager Conditional Target Dependencies

Amendment: During the implementation, Swift Package Manager code owners realized that it would be better to give target conditionals its own type instead of re-using the existing BuildSettingCondition type. This will allow controlling what exactly is exposed as API for targets and build settings conditionals as some conditions might not make sense in one or the other. A specialized type also allows independent evolution of the conditional APIs, however, the hope is to keep the APIs very similar in order minimize the impact on learning curve for new users.