Deduplicate output jar files

[turikhay]

Closes #117

New Paper remapper doesn't like duplicate entries in the plugin jar. I came up with a simple and a bit naive solution: we take output of shadowJar and create a new jar with DuplicatesStrategy.EXCLUDE set, and then replace the original one.

EDIT: We should avoid replacing original jar; it's better to just use dedupShadowJar output instead