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
