Gradle Multi-Module groovy 'Project' object not getting evaluated in helper files

I have a multi-module setup for a Java project with following structure.

mainApp
|--> core-module
|       |--> src
|       |--> build.gradle
|       |--> gradle.properties
|       
|--> lib-module
|       |--> src
|       |--> build.gradle
|       |--> gradle.properties
|--> lib-another-module
|       |--> src
|       |--> build.gradle
|       |--> gradle.properties
|--> settings.gradle
|--> build.gradle

And in my settings.gradle I've specified.

rootProject.name = 'mainApp'

include 'core-module'
include 'lib-module'
include 'lib-another-module'

in my mainApp/build.gradle file, I've written a helper as helper-tasks.gradle and imported that file in my $rootDir/build.gradle file as

apply from: "$rootDir/helpers/helper-tasks.gradle"

task('bundleJar', type: Jar) is a part of subprojects { subproject -> } closure and I'll be just calling ./gradlew bundleJar. Here expectations are it should iterate through each sub-project and collect the classes for my final bundle

in helper-tasks.gradle file I wrote a gradle tasks as below

task('bundleJar', type: Jar) {

    manifest {
        attributes 'Manifest-Version': "1.0"
        attributes 'Built-By': USER_NAME
        attributes 'Created-By': "Java Version: " + JAVA_VERSION
        attributes 'Implementation-Version': BUNDLE_VERSION + " Build " + GIT_COMMIT_HASH
        attributes 'Build-Timestamp': BUILD_TIME
    }
    baseName('lib-module')
    from project('lib-module').sourceSets.main.output.classesDirs
}

Here project('lib-module').sourceSets.main.output.classesDirs doesn't pick-up relative path of sub-module project $rootDir/lib-module/build/classes/java/main but still referring to "$rootDir/build/classes/java/main. Why? and how can I provide my settings to helper files?

There is a working example here on GitHub.

Consider the build.gradle at the root:

subprojects {
    apply plugin: 'java-library'
    apply from: "${rootProject.projectDir}/helpers/helper-tasks.gradle"
}

and helpers/helper-tasks.gradle:

task('bundleJar', type: Jar) {
    manifest {
        attributes 'Manifest-Version': "1.0"
        attributes 'Built-By': USER_NAME
        attributes 'Created-By': "Java Version: " + JAVA_VERSION
        attributes 'Implementation-Version': BUNDLE_VERSION + " Build " + GIT_COMMIT_HASH
        attributes 'Build-Timestamp': BUILD_TIME
    }
    baseName("${project.name}") 
    from project.sourceSets.main.output.classesDirs
}

Note that the name of the file now matches the project name. In the example, gradle lib-module:bundleJar will generate ./lib-module/build/libs/lib-module.jar with appropriate classes.

Similarly, gradle lib-another-module:bundleJar will generate ./lib-another-module/build/libs/lib-another-module.jar (with appropriate classes).