Add and enable enforcer rule which bans duplicated classes

Throughout the codebase there are modules which depend on artifacts which have duplicated classes (classes with same FQN). This is very much undesirable as at runtime one them just gets picked and there is no guarantee which one actually.

There is an enforcer rule which checks all the dependencies and fails the build in case there are duplicated classes, http://www.mojohaus.org/extra-enforcer-rules/banDuplicateClasses.html. We should add this one to the build. Either to default one, or to "full" profile in case the build time would be considerable slower.