Passing compilation flags to local Swift packages

[dmhts (Dima Hutsuliak)]

To bring a bit of historical context here:

1. There was already an idea of introducing package flavors
2. and a note from one of the (past?) SwiftPM maintainters:

@ddunbar

"Package Features": It would be very useful have something akin to Cargo's features mechanism in SwiftPM, so that packages could expose "knobs & switches" by which their clients could enable or control how the package was built. SwiftPM currently allows environment variables to be used for this purpose in the manifest, but it is a hack we really only allow because we don't have a proper feature for it. This is also somewhat related to more flexible build configurations or Bazel's configurable build attributes , although I see this as potentially orthogonal features.