Move custom build settings to Package.xcconfig

[MaxDesiatov]

This would allow us to remove .xcodeproj and to avoid conflicts as mentioned in #19. This would probably require moving benchmark baselines to a separate directory and to add a script that regenerates the project file and copies baselines to a correct location.

[MaxDesiatov]

Thinking about it more, it looks like .xcodeproj directory and other Xcode-related files are required by Carthage, so we may need to keep it in the repository 😞

[regexident]

If Carthage was to do a check for SPM projects with missing .xcodeproj files and added an intermediary step of running Swift package generate-xcodeproj it would work just fine. Alas the issue asking for support was deemed unimportant (due to inactivity). 😕
Carthage/Carthage#969

[MaxDesiatov]

looks like there's also semi-active Carthage/Carthage#1945