Swift Package Manager support

[vi4m]

Thanks for the awesome tool!
It would be great to have SPM support!

[miguelangel-dev]

After Apple's announcement, I think we should try to migrate from cocoapods to SPM. This issue will need:

1. [nef playground] create an initial playground, compatible with third-party libraries from SPM, or unless with an own solution.
2. [nef compile] SPM will be the default dependency manager.
3. [nef compile --podfile] we need to keep compatibility with cocoapods: for nef compiler, we can decide use cocoapods with the flat --podfile

After completing this issue, homebrew only will have one dependency, Xcode and macos (the only place where Xcode playgrounds are compatible for now)

[miguelangel-dev]

We have started the path to add compatibility to other dependency managers:

• Carthage (done in #89 #90 #91 - released in version 0.4)
• SPM (pending Apple fixes the SPM bug in Xcode - 47668990)

Swift Package Manager is not as easy as we can expect. Using the new Xcode 11 and SPM integrated does not resolve dependencies for a nested Playground as it does in CocoaPods or Carthage (Apple introduced it in Xcode 9).

We will start to decouple CocoaPods from nef, and integrate Carthage when it solves (or not) we will see how to integrate SPM in nef.

more information
https://developer.apple.com/documentation/xcode_release_notes/xcode_11_beta_5_release_notes

Xcode 11 beta 5
Known Issues
...
Playgrounds in a workspace can’t import targets from a Swift package. (47668990)
...

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https://developer.apple.com/documentation/xcode_release_notes/xcode_11_beta_6_release_notes

Xcode 11 beta 6

Playgrounds
Known Issues
...
Playgrounds in a workspace can’t import targets from a Swift package. (47668990)
...

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https://developer.apple.com/documentation/xcode_release_notes/xcode_11_release_notes

Xcode 11 GM - bug #47668990 not fixed