Generate javadoc for picocli.jar with java 9+

[remkop]

As a follow-up to #711, consider using Java 9+ to generate Javadoc for the root project.

• The Java 8 doclet sometimes complains it cannot find references to deeply nested inner classes
• The Java 9 doclet generates Javadoc with a Search textbox in upper right of the page
• I haven't yet figured out how to get it to generate frames by default

Related:

def exportedProjects= [
        ":",
        ":picocli-codegen",
        ":picocli-groovy",
        ":picocli-shell-jline2",
        ":picocli-shell-jline3",
        ":picocli-spring-boot-starter",
]

task alljavadoc(type: Javadoc) {
    source exportedProjects.collect { project(it).sourceSets.main.allJava }
    classpath = files(exportedProjects.collect { project(it).sourceSets.main.compileClasspath })
    destinationDir = file("${buildDir}/docs/javadoc")
}

[remkop]

Closing: superceded by #1298.