rename .dvc file to .lock file

[skshetry]

We have two concepts now: .dvc file and .lock file. It'd be good to have consistent set of files: dvc.lock and dvc.yaml. The files could be named .lock (it could collide with other .lock files eg: Pipfile.lock, etc. so maybe .dvc.lock).

[efiop]

.lock is a temporary file that we can live without, which is not the case for dvc added and dvc imported files right now. But even just when talking about renaming, .dvc.lock is the only option that is visually identifiable as something that belongs to dvc, at which point it is not clear why we would just make the suffix longer with no clear benefit. And right now .dvc belongs to data management, while dvc.yaml and dvc.lock to pipelines, which are useful to separate from each other.

[skshetry]

Having a single kind of format would be better to explain others conceptually. And, dvc.lock and .dvc files are the same thing with different file extension. Though, I did forget that it'd collide with other .lock files when making this issue.

[efiop]

@skshetry Actually they are not the same thing even anymore. E.g. .lock is multistage, but .dvc isn't. And the differences will be bigger and bigger as we go, as we are no longer limited by them (dvc run and dvc add .dvc-files) being the same thing. We used to have that in 0.x as you remember, and even though it was nice that it was the same format, but it was actually confusing in the real world, as data management and pipeline management didn't mix together nicely and we always had to explain all the corner cases (e.g. dvc add is a corner case of dvc run with no command, no deps), hence why we've moved to the current approach.