Mark buildSettingsDescription as @_spi in Target.swift (#7542)

rdar://127782783

Author: MaxDesiatov

Sources/Build/BuildDescription/ProductBuildDescription.swift

@@ -13,6 +13,7 @@
 import Basics
 import PackageGraph
 
+@_spi(SwiftPMInternal)
 import PackageModel
 
 import OrderedCollections

Sources/PackageModel/Target/Target.swift

@@ -235,7 +235,8 @@ public class Target: PolymorphicCodableProtocol {
     /// The build settings assignments of this target.
     public let buildSettings: BuildSettings.AssignmentTable
 
-    package let buildSettingsDescription: [TargetBuildSettingDescription.Setting]
+    @_spi(SwiftPMInternal)
+    public let buildSettingsDescription: [TargetBuildSettingDescription.Setting]
 
     /// The usages of package plugins by this target.
     public let pluginUsages: [PluginUsage]