safe `Dvcfile.dump()`

[skshetry]

Initially, the idea was not to touch dvc.yaml at all, but as we are doing it quite often, it's better to protect dvc.lock rather than dvc.yaml by default.
So, we could change it to the following default:

def dump(pipeline=True, lock=False):
    pass

The only place where we need to update lock are dvc repro and dvc run, at least atm.

Originally posted by @skshetry in #3994 (comment)

[efiop]

@skshetry Is this something that you are tackling right now(judging by the assignee)? If not, could you please elaborate a bit more? Also, please feel free to set appropriate labels.

[skshetry]

I'm not working right now, but something that I'd like to tackle next.
#3994 (comment) should have enough context about the issue.

[efiop]

@skshetry What about labels then?