Add Swift Package Manager as new opt-in feature for iOS and macOS (#146256)

This PR adds initial support for Swift Package Manager (SPM). Users must opt in. Only compatible with Xcode 15+.

Fixes https://github.com/flutter/flutter/issues/146369.

## Included Features

This PR includes the following features:
* Enabling SPM via config 
`flutter config --enable-swift-package-manager`
* Disabling SPM via config (will disable for all projects) 
`flutter config --no-enable-swift-package-manager`
* Disabling SPM via pubspec.yaml (will disable for the specific project)
```
flutter:
  disable-swift-package-manager: true
```
* Migrating existing apps to add SPM integration if using a Flutter plugin with a Package.swift
  * Generates a Swift Package (named `FlutterGeneratedPluginSwiftPackage`) that handles Flutter SPM-compatible plugin dependencies. Generated package is added to the Xcode project.
* Error parsing of common errors that may occur due to using CocoaPods and Swift Package Manager together
* Tool will print warnings when using all Swift Package plugins and encourage you to remove CocoaPods

This PR also converts `integration_test` and `integration_test_macos` plugins to be both Swift Packages and CocoaPod Pods.

## How it Works
The Flutter CLI will generate a Swift Package called `FlutterGeneratedPluginSwiftPackage`, which will have local dependencies on all Swift Package compatible Flutter plugins.  

The `FlutterGeneratedPluginSwiftPackage` package will be added to the Xcode project via altering of the `project.pbxproj`. 

In addition, a "Pre-action" script will be added via altering of the `Runner.xcscheme`. This script will invoke the flutter tool to copy the Flutter/FlutterMacOS framework to the `BUILT_PRODUCTS_DIR` directory before the build starts. This is needed because plugins need to be linked to the Flutter framework and fortunately Swift Package Manager automatically uses `BUILT_PRODUCTS_DIR` as a framework search path.

CocoaPods will continue to run and be used to support non-Swift Package compatible Flutter plugins.

## Not Included Features

It does not include the following (will be added in future PRs):
* Create plugin template
* Create app template
* Add-to-App integration
56 files changed
tree: 98c50b7859755a3a5b27f25b7cf9960b17ad2d8b
  1. .github/
  2. .vscode/
  3. bin/
  4. dev/
  5. examples/
  6. packages/
  7. .ci.yaml
  8. .gitattributes
  9. .gitignore
  10. analysis_options.yaml
  11. AUTHORS
  12. CODE_OF_CONDUCT.md
  13. CODEOWNERS
  14. CONTRIBUTING.md
  15. dartdoc_options.yaml
  16. flutter_console.bat
  17. LICENSE
  18. PATENT_GRANT
  19. README.md
  20. TESTOWNERS
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