Make log4j-core a Java 9 module

Log4j-core is currently an automatic module.

By making it a real module the Java platform can help enforce the distinction between internal and exported classes.

This epic links together tickets related to this goal. I expect that most of the work will be about:

• moving external dependencies out of the core module 
• reorganizing packages and classes into internal and exported packages. Other Log4j modules and custom plugins can depend on the exported packages but not the internal ones.