Fix SPM support

[M0rtyMerr]

Current Package.swift was broken, so I fixed it

[guillaumealgis]

While I'm not a huge fan of hiding all the UIKit code from SwiftPM to make it accept the package, I don't see any other way right now.
Until SwiftPM implements platform restrictions for packages (and that's probably not coming soon, I could not find any discussion on this topic) I'd say let's roll with that.

So +1, clever trick and seems to work well, thank you @MortyMerr !
I'd love to see this PR merged ☺️

[AliSoftware]

Thanks a lot for this PR 👍
I'm gonna merge this as it as it's usable but I wonder if in a subsequent PR it would make sense to add some #error "This library only supports UIKit" in the #else branch 🤔 😉

[M0rtyMerr]

I guess it should be obvious from library description)

_{Sent with GitHawk}

[M0rtyMerr]

Thanks a lot for attention

_{Sent with GitHawk}

[guillaumealgis]

I'm gonna merge this as it as it's usable but I wonder if in a subsequent PR it would make sense to add some #error "This library only supports UIKit" in the #else branch 🤔 😉

@AliSoftware Haven't tested it, but I suppose it would defeat the purpose of this PR as SPM would choke on the #error when trying to compile the package (it seems Xcode's SPM integration tries to compile the dependencies with macOS as the target, hence the UIKit error).