refactor: 💥 header files with the entire system definitions moved up in the directory tree

example/unmanned_aerial_vehicle.cpp

@@ -29,9 +29,9 @@ import mp_units;
 #else
 #include <mp-units/framework/quantity_point.h>
 #include <mp-units/ostream.h>
-#include <mp-units/systems/international/international.h>
+#include <mp-units/systems/international.h>
 #include <mp-units/systems/isq/space_and_time.h>
-#include <mp-units/systems/si/unit_symbols.h>
+#include <mp-units/systems/si.h>
 #endif
 
 using namespace mp_units;