Don't sort ProjectChecker messages by line number

[Brownies]

ProjectChecker messages are sorted here and because of the comparison function for messages they get sorted by line number, column and then rule id.

I don't think this makes sense specifically for rules that operate on the whole project. I would prefer the checker output didn't get sorted at all in run_project_checks() and so the order would be up to the checker. But sorting by source path first would also be an improvement.

[bhirsz]

Good point. At this point I would even consider current behaviour as a bug.