Support Swift Packages

[toshi0383]

#606

I personally needed this feature, so implemented.
Looks like Yonas already has an implementation, so feel free to just close this one, but otherwise people can use this branch for now.

If you want me to keep going, I will clean up the dirty codes and add tests etc.👌

project spec consideration

I chose yml format to be rather simple than elegant, using XCRemoteSwiftPackageReference directly.

Sample project.yml:

name: App
targets:
  App:
    type: application
    platform: iOS
    dependencies:
      - swiftpm:
        repositoryURL: "https://github.com/tuist/xcodeproj"
        versionRequirement:
          kind: upToNextMajorVersion
          minimumVersion: 7.0.0

See: https://github.com/tuist/xcodeproj/blob/58c6acf1111b92a9acf633359a10249cf7940045/Sources/xcodeproj/Objects/SwiftPackage/XCRemoteSwiftPackageReference.swift

swiftpm process

SwiftPM's package resolving and fetching process begins when you open the generated xcodeproj with Xcode11beta.

[yonaskolb]

Thanks for the PR @toshi0383. I have indeed gone another approach, most notably separating the definition of a package from linking to it. This allows for multiple targets to use the same package. Coming back from a break so I'll open this soon

[yonaskolb]

Swift Package support has been added here #624

Closing this PR now