Add Swift Package Manager compatibility

Current status:

• SwiftPM is currently available as a experimental feature: https://docs.flutter.dev/packages-and-plugins/swift-package-manager
• Iterating on SwiftPM by adding the Flutter framework as a local override: [SwiftPM] ☂ Adopt local package override for the Flutter framework SwiftPM dependency #166486

See Github subtasks for progress and planned work

Tracking of additional features and bugs:

• [tool] Always generate the Package.swift file #153448
• No way to resolve name collisions with SwiftPM with Flutter #148424
• Could not find BuildActionEntries, when building with swift package manager and flavors #163086
• Plugin with only Swift Package Manager support raise error on flutter pub get from a non MacOS machine #162594
• flutter <build> <ios> regenerates plugins multiple times? #157391
• [SwiftPM] Support conditional compilation in plugins #161182
• [SwiftPM] Improve the error message if a plugin requires a higher deployment target version #165420
• Do not show Swift Package Manager warnings when targeting non-iOS or macOS #185775
• macOS build failures are not parsed for common causes #144397
  • Common SwiftPM errors are not currently parsed for macOS due to this issue. Once resolved, we should parse for common errors same as iOS
• [SwiftPM] Xcode build does not update the generated package's supported platforms #162196
• [SwiftPM] Plugin not found right after adding #162399
• Improve the error if plugins require CocoaPods but CocoaPods is not installed #163284
• [SwiftPM] Adding 'arm64' to excluded simulator architectures breaks plugins #160329
• Ability to disable Swift Package Manager per plugin #146203
• [packages] Consider re-adding SwiftPM .modulemaps (see Remove use of modulemaps in flutter/packages when using SwiftPM #148572 (comment))
• Upload .gitkeep files/empty directories when publishing dart-lang/pub#4262
  • Workaround in documentation (using !.gitkeep in .gitignore)

Resources

Design doc:

• flutter.dev/go/swift-package-manager-plugins-v2 (outdated, see Add Swift Package Manager as new opt-in feature for iOS and macOS #146256)
• flutter.dev/go/swift-package-manager-plugins-v3

Original Github Issue: #33850