Avoid relative path in Header file includes with Cmake

I have a project structured like below -

app
|--src
|  |---module
|  |   |---file1.cpp
|  |   |---file1.hpp
|  |---file2.cpp
|  |---file2.hpp
|  |---main.cpp
|---CMakeLists.txt

CMakeLists.txt

cmake_minimum_required(VERSION 3.15)    
add_executable(testApp)
target_include_directories( testApp INTERFACE src)
        
target_sources(
src/file2.cpp
src/file2.cpp
src/module/file1.hpp
src/module/file1.cpp
src/main.cpp
)

file1.cpp

#include"../file2.hpp"

//....some code.....

Question: In file1.cpp, I am including #include"../file2.hpp" giving a relative path to file2.hpp. How do I avoid this ??

I am including this path to CMakeLists.txt

target_include_directories( testApp INTERFACE src)

But when I do #include "file2.hpp" I get a file not found error. How do I include the src folder to my project so that I can use the headers file directly ??

target_include_directories — CMake 3.27.0-rc4 Documentation

> The INTERFACE, PUBLIC and PRIVATE keywords are required to
> specify the
> scope
> of the following arguments. PRIVATE and PUBLIC items will
> populate the
> [INCLUDE_DIRECTORIES](https://cmake.org/cmake/help/latest/prop_tgt/INCLUDE_DIRECTORIES.html#prop_tgt:INCLUDE_DIRECTORIES
> "INCLUDE_DIRECTORIES") property of <target>. PUBLIC and
> INTERFACE items will populate the [INTERFACE_INCLUDE_DIRECTORIES](https://cmake.org/cmake/help/latest/prop_tgt/INTERFACE_INCLUDE_DIRECTORIES.html#prop_tgt:INTERFACE_INCLUDE_DIRECTORIES
> "INTERFACE_INCLUDE_DIRECTORIES") property of <target>. The
> following arguments specify include directories.

INTERFACE_INCLUDE_DIRECTORIES — CMake 3.27.0-rc4 Documentation

> When target dependencies are specified using
> [target_link_libraries()](https://cmake.org/cmake/help/latest/command/target_link_libraries.html#command:target_link_libraries
> "target_link_libraries"), CMake will read this property from all
> target dependencies to determine the build properties of the consumer.

So basically INTERAFACE means that only target which links to given target will have this include directories. Current target build will not be impacted.

Just change this to PUBLIC:

target_include_directories( testApp PUBLIC src)