Xcode creates a .build directory for apps that use the Swift Package Manager. This contains build artifacts and should not be check-in to version control. We should update .gitignore files:

1. Update the app template & add a migration to the Flutter tool

2. If necessary, also update the plugin template.

   flutter channel main
   flutter config --enable-swift-package-manager
   
   flutter create -t plugin --platforms=ios foo
   cd foo
   git init ; git add . ; git commit -m "Init"
   

   In Xcode, open the directory that contains Package.swift: ios/foo/

   $ git status
   Untracked files:
   (use "git add <file>..." to include in what will be committed)
    ios/foo/.swiftpm/
   

3. Update the flutter/packages repo

4. Update all sample projects' .gitignore files.

Part of #126005