Add support for Swift 5.6 package description format

[MaxDesiatov]

Starting with Swift 5.6 swift package dump-package outputs JSON in format that's incompatible with Swift 5.5 declarations. Ideally, we should be able to support both Swift 5.5 and 5.6 package description formats in carton.

[SDGGiesbrecht]

Is this something I can help with? It sounds like it is just a matter of adapting the decoding of DumpedManifest here for changes to the encoding of Manifest here. Or am I missing something? (I am also not very familiar with your testing arrangement.)

[MaxDesiatov]

That's largely correct, but I personally would like 5.5 and earlier Swift versions to be supported as well. One possible approach would be to keep these as separate model types, say DumpedManifest_v5_5 and DumpedManifest_v5_6 and decode into these conditionally, based on the detected toolchain version.

[SDGGiesbrecht]

I will give it a shot. I am new to your code base though, and probably not as fast as someone more familiar with it. So if it is already on someone’s to‐do list, then feel free to race ahead and not wait for me.

[SDGGiesbrecht]

This is not as easy as I thought. For one, SwiftPM needs to be updated, because the Manifest type you are using is unable to represent everything dumped by 5.6. But some of its new component types are neither Decodable nor have public initializers. I think you need to roll you own Manifest type, but if you need it to interact with other SwiftPM APIs, then...