src folder in sketch structure and include library from subfolder
I could include header files in subfolder after including root header file.

#include &lt;TB6612_Dual.h&gt;
#include &lt;TB6612_Dual/tb6612_dual.h&gt;


TB6612_Dual
└── src
    ├── TB6612_Dual.h &lt;&lt; Add this empty file.
    └── TB6612_Dual
        ├── tb6612_dual.cpp
        └── tb6612_dual.h


Examples:


utils_asukiaaa/examples/string/string.ino
utils_asukiaaa/src/utils_asukiaaa/string.h
utils_asukiaaa/src/utils_asukiaaa/string.cpp
utils_asukiaaa/src/utils_asukiaaa.h