External Cartfile specifications for dependencies about carthage

[jspahrsummers]

Tentatively dropping this into the milestone, mostly just so I don't overlook it later. Let me know if you think it should be pulled out.

[alanjrogers]

@jspahrsummers Great idea, I think this is important for an initial ship so that people can start using it without having to wait for library maintainers to update.

[robrix]

Agreed.

[jspahrsummers]

I've kinda 180ºd on this. I can't think of a good syntax off of the top of my head, and this introduces some additional complexity around locating those external references.

Since there is a workaround available (forking), I'm interested in punting this in the interest of minimizing scope. Please speak up if you think I'm crazy. 😉

[jspahrsummers]

This hasn't really been a problem in practice—probably partly thanks to Carthage's interoperability with other dependency management solutions (CocoaPods, submodules, subtrees).

After all, this only applies to dependencies that themselves have dependencies, which is a fairly small group.