Sink the module dependency scanner down into the Serialization library

lib/Frontend/CMakeLists.txt

@@ -8,7 +8,6 @@ add_swift_host_library(swiftFrontend STATIC
   Frontend.cpp
   FrontendInputsAndOutputs.cpp
   FrontendOptions.cpp
-  ModuleDependencyScanner.cpp
   ModuleInterfaceBuilder.cpp
   ModuleInterfaceLoader.cpp
   ModuleInterfaceSupport.cpp

lib/Serialization/CMakeLists.txt

@@ -1,6 +1,7 @@
 add_swift_host_library(swiftSerialization STATIC
   Deserialization.cpp
   DeserializeSIL.cpp
+  ModuleDependencyScanner.cpp
   ModuleFile.cpp
   Serialization.cpp
   SerializedModuleLoader.cpp