find_package(): Add policy to remove the FindQt module

Removing FindQt.cmake gives Qt upstream a path forward to export its
own QtConfig.cmake files which can be found by find_package()
without having to explicitly specify CONFIG. Projects that still
want to use Qt3/4 can call find_package(Qt[34]), include(FindQt),
or add FindQt.cmake to their CMAKE_MODULE_PATH.

Source/cmFindPackageCommand.h

@@ -4,6 +4,7 @@
 #define cmFindPackageCommand_h
 
 #include "cmConfigure.h" // IWYU pragma: keep
+#include "cmPolicies.h"
 
 #include "cm_kwiml.h"
 #include <cstddef>
@@ -148,6 +149,8 @@ private:
   };
   std::map<std::string, OriginalDef> OriginalDefs;
 
+  std::map<std::string, cmPolicies::PolicyID> DeprecatedFindModules;
+
   std::string Name;
   std::string Variable;
   std::string Version;