Provide relative path to jpackage --main-jar. Fixes #1578

[mcpiroman]

Per jpackage doc:

--main-jar main jar file The main JAR of the application; containing the main class (specified as a path relative to the input path).

Either --module or --main-jar option can be specified but not both.

Tested only on Windows.

Fixes #1578

[Omico]

@mcpiroman This works for me. Thanks!

[jasonsparc]

@AlexeyTsvetkov @igordmn I just wanted to know, when will this PR be approved? I know compose multiplatform is still in alpha, but it would really be nice if this PR gets approved. Currently, the main issue is that it's leaking sensitive information (i.e., the build machine's path) when people look into the app/*.cfg file (which is present even through the generated installer).

It's harmless at runtime though, given that the actual main jar is already fed via the --input option, so I wish we could just omit the --main-jar option, but sadly, jpackage requires it.

Putting your project in a directory with a non-sensitive information is a workaround, but I wish we could just build our projects anywhere without worrying about this. Also, very likely, a lot of people aren't even aware of this issue.

[AlexeyTsvetkov]

Merged the PR manually with some additional changes on top