Adding Debug source set in java library module in android studio

In my project I have a java library module in which I need to add a debug folder to add some source file that will be used only in debug build.

Gradle file for java library is as follows:

apply plugin: 'java-library'
apply plugin: 'kotlin'

dependencies {
    implementation kotlinx.stdlib //defined in deps.gradle

    implementation junit.core //defined in deps.gradle
}

sourceCompatibility = "8"
targetCompatibility = "8"

For now I have following structure:

app

my-java-lib
  |
  +--- src
        |
        +--- main
        |      |
        |      +--- java
        |
        +--- test
               |
               +--- java

When I add debug\java folder under src Android studio does not recognized as source folder.

How can I make Android studio to recognize as source folder?