Duplicate check for feature files

[gvenzl]

Given that feature file partials can now be grouped in subfolders, we should have a duplicate feature file name check to make sure users don't overwrite feature files by accident by using the same name.
Technically, there will never be a duplicate feature but that doesn't mean an author may pick a shorter name that could collide with another author's file.
In the event of such a clash, the file shouldn't be overwritten but the build should be aborted, telling the author to resolve the conflict.