RFR: JDK-8231382: Use main class from main module if available

[Alexander Matveev]

Please review the jpackage fix for bug [1] at [2].

This is a fix for the JDK-8200758-branch branch of the open sandbox 
repository (jpackage).

- Main class from module will be used if main class is not specified in 
CLI. Main class from CLI will be preferred over main class specified in 
module attribute.

[1] https://bugs.openjdk.java.net/browse/JDK-8231382

[2] http://cr.openjdk.java.net/~almatvee/8231382/webrev.00/

Thanks,
Alexander