Add Sendable to FileRuleDescription

Author: Kohki Miki

Sources/PackageLoading/TargetSourcesBuilder.swift

@@ -563,11 +563,11 @@ public struct TargetSourcesBuilder {
 }
 
 /// Describes a rule for including a source or resource file in a target.
-public struct FileRuleDescription {
+public struct FileRuleDescription: Sendable {
     /// A rule semantically describes a file/directory in a target.
     ///
     /// It is up to the build system to translate a rule into a build command.
-    public enum Rule: Equatable {
+    public enum Rule: Equatable, Sendable {
         /// The compile rule for `sources` in a package.
         case compile