Publish Android sources jar on Gradle 5+

build.gradle

apply plugin: 'digital.wup.android-maven-publish'

group = 'com.github.User'
version = '0.1'

task sourceJar(type: Jar) {
    from android.sourceSets.main.java.srcDirs
    classifier "sources"
}

publishing {
    publications {
        mavenAar(MavenPublication) {
            groupId group
            artifactId 'library'
            version version
            artifact(sourceJar)
            from components.android
        }
    }
}