Make picocli-codegen an explicitly declared named JPMS module

[remkop]

It should be possible to make the picocli-codegen artifact a full-blown named Java 9 JPMS module, since this subproject has no dependencies other than the picocli artifact.

// module-info.java
module info.picocli.codegen {
    requires info.picocli;
    requires java.base;
    requires java.compiler;
    requires java.logging;

    exports picocli.codegen.annotation.processing;
    exports picocli.codegen.aot.graalvm;
    exports picocli.codegen.aot.graalvm.processor;
    exports picocli.codegen.docgen.manpage;
}

What makes this tricky is that we need to build the picocli project with Java 8 in order to get Java 5 bytecode for the main picocli.jar artifact. We cannot use Java 9, because from Java 9 onwards the javac tool no longer supports -source 1.5 and -target 1.5 (the minimum becomes 1.6, and from Java 11 the minimum is 1.7).

To compile the module-info.java files with Java 8, we use the https://github.com/beryx/badass-jar-plugin Gradle plugin.

There was some issue with beryx/badass-jar 1.2.0 that meant building a modular picocli jar worked, but a modular picocli-codegen jar failed. It looks like the new 2.0.0 version does not have that problem.

Result:

> %JAVA_11_HOME%\bin\jar -v -d  -f C:\Users\remko\IdeaProjects\picocli3\picocli-codegen\build\libs\picocli-codegen-4.7.0-SNAPSHOT.jar --release 9
releases: 9

info.picocli.codegen@4.7.0-SNAPSHOT jar:file:///C:/Users/remko/IdeaProjects/picocli3/picocli-codegen/build/libs/picocli-codegen-4.7.0-SNAPSHOT.jar/!META-INF/versions/9/module-info.class
exports picocli.codegen.annotation.processing
exports picocli.codegen.aot.graalvm
exports picocli.codegen.aot.graalvm.processor
exports picocli.codegen.docgen.manpage
requires info.picocli
requires java.base mandated
requires java.compiler
requires java.logging

> %JAVA_11_HOME%\bin\jar -v -d  -f C:\Users\remko\IdeaProjects\picocli3\build\libs\picocli-4.7.0-SNAPSHOT.jar --release 9
releases: 9

info.picocli@4.7.0-SNAPSHOT jar:file:///C:/Users/remko/IdeaProjects/picocli3/build/libs/picocli-4.7.0-SNAPSHOT.jar/!META-INF/versions/9/module-info.class
exports picocli
requires java.base mandated

This is part of an effort to generate modular javadoc (#1298).