[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 #381).
• Test cases for the functionalities.