refactor: Refactored the library file tree

Resolves #249

example/foot_pound_second.cpp

@@ -22,17 +22,17 @@
 
 #include <units/concepts.h>
 #include <units/format.h>
-#include <units/physical/si/fps/derived/density.h>
-#include <units/physical/si/fps/base/length.h>
-#include <units/physical/si/fps/base/mass.h>
-#include <units/physical/si/fps/derived/power.h>
-#include <units/physical/si/fps/derived/speed.h>
-#include <units/physical/si/fps/derived/volume.h>
-#include <units/physical/si/base/length.h>
-#include <units/physical/si/base/mass.h>
-#include <units/physical/si/derived/power.h>
-#include <units/physical/si/derived/speed.h>
-#include <units/physical/si/derived/volume.h>
+#include <units/physical/si/fps/density.h>
+#include <units/physical/si/fps/length.h>
+#include <units/physical/si/fps/mass.h>
+#include <units/physical/si/fps/power.h>
+#include <units/physical/si/fps/speed.h>
+#include <units/physical/si/fps/volume.h>
+#include <units/physical/si/length.h>
+#include <units/physical/si/mass.h>
+#include <units/physical/si/power.h>
+#include <units/physical/si/speed.h>
+#include <units/physical/si/volume.h>
 #include <units/quantity.h>
 #include <iostream>