run: warning/exception when dependency is a dvc-file

It doesn't make a lot of sense to depend on a dvc-file, however, the interface is not intuitive enough to prevent this.

If we want to allow this behavior, it should be with a warning.

[efiop]

As a first step we might just add a check that dependency is not a dvc file and throw an error. Having it as a feature also sounds quite neat, so one could do -d my.dvc which would mean "depend on all outputs of my.dvc". Not sure if that feature is desirable though, since it might be used by accident causing some obscure bugs in the pipeline.

[efiop]

Discord context https://discordapp.com/channels/485586884165107732/485596304961962003/605892459461083140

ohh, @efiop, that sounds really neat! never thought about it, and it make sense since "the output of a stage become the input of the next one".