cxxmodules: generate synthetic targets as an initial pass

We need to be able to construct BMIs that will be usable from the client
modules for the target importing the module, so create BMI-only
compilation rules for `IMPORTED` targets to create these BMIs.

Source/cmFileSet.h

@@ -41,6 +41,8 @@ public:
   const std::string& GetType() const { return this->Type; }
   cmFileSetVisibility GetVisibility() const { return this->Visibility; }
 
+  void CopyEntries(cmFileSet const* fs);
+
   void ClearDirectoryEntries();
   void AddDirectoryEntry(BT<std::string> directories);
   const std::vector<BT<std::string>>& GetDirectoryEntries() const