RFR: JDK-8232723: make the resource file main.wxs over-ridable

[Andy Herrick]

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).

use createResource() to get an over-ridable resource.

[1] https://bugs.openjdk.java.net/browse/JDK-8232723

[2] http://cr.openjdk.java.net/~herrick/8232723/webrev.01

/Andy