Bundle.module should be a constant. (#5737)

rdar://94565594

(cherry picked from commit 208f929)

Author: neonichu

Sources/Build/BuildPlan.swift

@@ -785,7 +785,7 @@ public final class SwiftTargetBuildDescription {
         import class Foundation.Bundle
 
         extension Foundation.Bundle {
-            static var module: Bundle = {
+            static let module: Bundle = {
                 let mainPath = \(mainPathSubstitution)
                 let buildPath = "\(bundlePath.pathString.asSwiftStringLiteralConstant)"