Improve duplicate JAR resolving

Just a minor code style tweak that makes the resolving IMHO more obvious und more functional.

In addition, now on conflicts there is printed a single warning (per `noVersionJarName`) like

~~~
[warn] Version conflict on com.mycompany. Using 1.2.3 (found 1.0, 1.2.3-alpha, 1.2.3)
~~~

instead of multiple warnings for each found conflicting version.