move macOS 15 guard to the execute() function only

Author: sebsto

Plugins/AWSLambdaPackager/Plugin.swift

@@ -13,10 +13,8 @@
 //===----------------------------------------------------------------------===//
 
 import PackagePlugin
-import Synchronization
 import Foundation
 
-@available(macOS 15.0, *)
 @main
 struct AWSLambdaPackager: CommandPlugin {
     func performCommand(context: PackagePlugin.PluginContext, arguments: [String]) async throws {

Plugins/AWSLambdaPackager/PluginUtils.swift

@@ -17,6 +17,7 @@ import PackagePlugin
 import Synchronization
 import Foundation
 
+@available(macOS 15.0, *)
 struct Utils {
     @discardableResult
     static func execute(