[MJMOD-27] Implement unpack goal

**[Alexander Kriegisch](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=kriegaex)** opened **[MJMOD-27](https://issues.apache.org/jira/browse/MJMOD-27?redirect=false)** and commented

There has not been another alpha release for 2.5 years (2017/09 to 2020/05), so I am not sure if this project is dead or alive. But in case development is still active, I would like to have an option to not only to create a JMOD but also to **unpack** one. Maybe this could also be implemented in Maven Assembly. I am not sure where it would fit best, but I think here.

Background:
* Sometimes I want to instrument JRE/JDK classes using ASM, ByteBuddy, Javassist or similar byte code engineering tools during build time in order to then either prepend the resulting JAR file to the boot classpath (Java <= 8) or to use `--patch-module` (Java >= 9).
* In order to do that in Java 9+ environments, I have to use JDK command line tools like _jmod.exe_ (unpack single JMOD from _jmods_ directory) or _jimage.exe_ (unpack _lib/modules_).
* The result is a partly scripted build, using a Beanshell script via Build Helper or something like Antrun.

The benefit of having an unpack feature accessible via a Maven plugin is obvious, I think. Who likes scripted builds?

---

**Affects:** 3.0.0-alpha-1

Start by examining the existing Maven JMod Plugin goal that creates JMODs and the JDK jmod/jimage unpack commands described in the issue. Verify the desired unpacked module contents against those tools; no project files or tests are named, so the relevant entry point and test coverage must be located first.

Written by the indexing model from the issue text.