Option to not place the runnableJar into Java/ with Mac distributions

[keastrid]

I'm submitting a…

• bug report
• feature request
• other

Short description of the issue/suggestion:
Option to place/leave the runnableJar in Contents/Resources instead of Contents/Resources/Java.

What is the current behavior?
Places the runnableJar in the Java folder.

What is the motivation / use case for changing the behavior?
AstroImageJ's update process downloads and renames "ij.jar" (the runnableJar), and relies on it being in a specific place. Running the version in the Java folder interferes in the update process.

Please tell us about your environment:

• JavaPackager version: 1.3.0
• OS version: Windows 10
• JDK version: 14
• Build tool:
  • Maven
  • Gradle

Other information (e.g. related issues, suggestions how to fix, links for us to have context)