[PackageLoading] Fix public declaration on a property.

 - The way this was being done currently is actually just relying on a compiler
   bug, and fails in release builds (see SR-2315).

(cherry picked from commit 5b61a83)

Author: ddunbar

Sources/PackageLoading/ModuleMapGenerator.swift

@@ -16,28 +16,29 @@ public let moduleMapFilename = "module.modulemap"
 
 /// A protocol for modules which might have a modulemap.
 protocol ModuleMapProtocol {
-
     var moduleMapPath: AbsolutePath { get }
 
     var moduleMapDirectory: AbsolutePath { get }
 }
 
-extension ModuleMapProtocol {
-    public var moduleMapPath: AbsolutePath {
-        return moduleMapDirectory.appending(component: moduleMapFilename)
-    }
-}
-
 extension CModule: ModuleMapProtocol {
     var moduleMapDirectory: AbsolutePath {
         return path
     }
+
+    public var moduleMapPath: AbsolutePath {
+        return moduleMapDirectory.appending(component: moduleMapFilename)
+    }
 }
 
 extension ClangModule: ModuleMapProtocol {
     var moduleMapDirectory: AbsolutePath {
         return includeDir
     }
+
+    public var moduleMapPath: AbsolutePath {
+        return moduleMapDirectory.appending(component: moduleMapFilename)
+    }
 }
 
 /// A modulemap generator for clang modules.