I could include header files in subfolder after including root header file.
#include <TB6612_Dual.h>
#include <TB6612_Dual/tb6612_dual.h>
TB6612_Dual
└── src
├── TB6612_Dual.h << Add this empty file.
└── TB6612_Dual
├── tb6612_dual.cpp
└── tb6612_dual.h
Example (my project)Examples: utils_asukiaaa/examples/string/string.ino