Make `ios_check_beta_deps` validate Swift Package Manager dependencies as well

[mokagio]

I looked for mentions of this necessity elsewhere but couldn't find any. I'm surprised this hasn't been tracked yet so I guess I just haven't searched well enough. If that's the case, we can close this and add a link. Otherwise...

As we progressively move from CocoaPods to SwiftPM as the dependency manager for our apps, we ought to update our tooling accordingly. In particular, ios_check_beta_deps should list Swift packages that point to non-stable versions in the same way it lists Pods.

This should be a matter of parsing the Package.resolved JSON looking for entries pointing to branches (for which we also ought to have a dedicated Dangermattic check to prevent this, Automattic/dangermattic#85) or commits.