Composite build problem

Hello,
What I would like is to have the following project structure:

composite-build
- application (uses gradle plugin from 'library/gradle-plugin')
- library
    - gradle-plugin (references some utility classes from 'utils')
    - utils

My problem is that the Gradle plugin is not published to any repository, I just tried to substitute the dependency locally by using:

includeBuild("library") {
    dependencySubstitution {
        substitute(module("com.example.library:gradle-plugin")).with(project(":gradle-plugin"))
     }
}

But it doesn’t seem to work, it always fails with the message:

A problem occurred configuring project ':application'.
> Could not resolve all artifacts for configuration ':application:classpath'.
   > Could not find com.example.library:gradle-plugin:1.0.0-SNAPSHOT.
     Searched in the following locations:
       - https://plugins.gradle.org/m2/com/example/library/gradle-plugin/1.0.0-SNAPSHOT/maven-metadata.xml
       - https://plugins.gradle.org/m2/com/example/library/gradle-plugin/1.0.0-SNAPSHOT/gradle-plugin-1.0.0-SNAPSHOT.pom
     Required by:
         project :application

Because the project is slightly complex (even if it is a very dummy one ), I shared it on Github: https://github.com/NorbertSandor/composite-build

Do you have an idea what is wrong with it?
Thanks for your help in advance.