Allow manual specification of main dist jar

[rnett]

Currently the main distribution jar used to look up module uses is hard coded in Util.getMainDistJarFile. I would like to be able to specify this, in particular to point to a proguarded jar.

This also leads to issues when using shadow, since the task will expect a *-all.jar without declaring a dependency on shadowJar to build it.

[satsen]

This is very important to have. There is a mainJar property in the jpackage block, but not in the runtime one.