Add swift project manager project description support

[uber-mahyar2]

It will make the buck dependency optional for this project. It will remove potential support for objective-c modules unless the swift project manager adds it later.

[uber-mahyar2]

SPM has ObjC support now. An issue with SPM that I've run into although is how you cannot specify an application target in SPM, application targets need to be xcode projects still. This project generator would need to create a graph of libraries with a root library. That should be mostly be fine since the build behavior should be the same.