Use --build-system experimental-async as a feature flag

Author: MaxDesiatov

Sources/Commands/SwiftBuildCommand.swift

@@ -140,7 +140,7 @@ public struct SwiftBuildCommand: AsyncSwiftCommand {
             throw ExitCode.failure
         }
 
-        if self.globalOptions.build.useAsyncBuildSystem {
+        if self.globalOptions.build.buildSystem == .experimentalAsync {
             let buildSystem = try swiftCommandState.createAsyncBuildSystem(
                 explicitProduct: options.product,
                 shouldLinkStaticSwiftStdlib: options.shouldLinkStaticSwiftStdlib,

Sources/CoreCommands/Options.swift

@@ -446,9 +446,6 @@ public struct BuildOptions: ParsableArguments {
     @Option(name: .customLong("build-system"))
     var _buildSystem: BuildSystemProvider.Kind = .native
 
-    @Flag(name: .customLong("experimental-async-build-system"), help: .hidden)
-    public var useAsyncBuildSystem: Bool = false
-
     /// The Debug Information Format to use.
     @Option(name: .customLong("debug-info-format", withSingleDash: true))
     public var debugInfoFormat: DebugInfoFormat = .dwarf

Sources/SPMBuildCore/BuildSystem/BuildSystem.swift

@@ -132,6 +132,7 @@ public struct BuildSystemProvider {
     public enum Kind: String, CaseIterable {
         case native
         case xcode
+        case experimentalAsync = "experimental-async"
     }
 
     public let providers: [Kind: any BuildSystemFactory]