[WIP] Fully support cross-compilation model based on destination.json

[stevapple]

• Deprecate swift.SDK configuration and introduce swift.destination.
• Read settings from existing destination.json file.
• Generate destination.json per workspace.
• Pass --destination <destination> on Swift tasks.

TODO:

• SourceKit-LSP support.
• Watch settings and regenerate destination.json automatically.
• Add a command to export destination.json.
• Watch destination.json and prompt to reload SourceKit-LSP.
• Gardening and refactoring.
• Builtin destinations to pick over (extending Add quick switcher between iOS and macOS targets #381).
• Test cases for the functionalities.

[adam-fowler]

@stevapple shall we close this given Cross Compilation Swift SDKs looks like it is about to be approved?

[stevapple]

@adam-fowler I think this needs a rework to match the V2 destination manifest. We still need a similar functionality and are still waiting for improved SK-LSP support for destination.

It's okay to close this specific PR, but we may need to keep it as a feature request.