Repackage META-INF/<module-name>.kotlin_module files into BOOT-INF/classes

[thorntonrp]

Issue #15890 was addressed to to repackage META-INF/.kotlin-module (with a dash), but the the suffix is supposed to be .kotlin_module with an underscore instead of a dash. The dash instead of the underscore was my bad from when I reported the issue. I apologize for the confusion.

Running jar tf against a kotlin project reveals that the correct suffix is kotlin_module.

[wilkinsona]

Gah, thanks for spotting this @thorntonrp and sorry that I didn't spot it when I implemented the fix. I'll close this one in favour of your PR.