Remove Clara from external folder

As Clara is no longer maintained as a separate project, the
implementation was moved to the internal subfolder of top-level
folder. This removes one folder and avoids potential user confusion.

Also simplified the convenience header checking script accordingly.

src/catch2/internal/catch_clara.hpp

@@ -23,7 +23,7 @@
 #pragma clang diagnostic ignored "-Wshadow"
 #endif
 
-#include <catch2/external/clara.hpp>
+#include <catch2/internal/catch_clara_upstream.hpp>
 
 #ifdef __clang__
 #pragma clang diagnostic pop