Current logic is extremely complex and messy, hiding a lot of bugs
inside. There are pretty much only 2 scenarios for stage creation:
1) `dvc add/import/etc` when dvcfile is created beside the output file
2) `dvc run` when dvcfile is created at the place of execution
both are transparently affected by `--fname` flag. Since we've removed
`--cwd` complexity already, there is no reason to generalize stage path
resolution for those two scenarios.