src folder in sketch structure and include library from subfolder My Arduino/PlatformIO libraries folder

libraries
└── cleaner_robot
    └── TB6612_Dual
        ├── tb6612_dual.h
        └── tb6612_dual.cpp


I used this code to innclude it:

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


But the compilation has errors like "undefinded variables, undefinded reference,...etc" 

This is my sketch folder

Cleaner_Robot
└── src
    └── TB6612_Dual
        ├── tb6612_dual.cpp
        └── tb6612_dual.h


If i use this code, all is ok

#include "src/libs/TB6612_Dual/tb6612_dual.h"


So the question is how to include if header file is in a sub folder of a folder in libraries folder of Arduino/PlatformIO ?, i has searched for many things, but cant help. 
I also read about src folder allowed in Arduino, but where can find an official announcement/manual for this structure ?, cant find much about this

Thanks  :)
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