import: support importing the whole repo

From #2012 Support importing the whole repo with dvc import myrepo. This is going to be similar to specifying multiple datas within the repo, e.g. dvc import myrepo data1 data2, but we will have to collect it ourselves for the whole repo. Probably worth putting into a directory by default. E.g. dvc import https://github.com/iterative/dvc -> create ‘dvc’ directory in which data files will be places same way they are placed in the original repo.

[efiop]

@dberenbaum Thanks for the heads up! Indeed, downgraded the priority. The underlying functionality is actually already present (repofs and dvcfs are both work with repo roots just fine and we are able to stage any directory), so just a matter of making sure import/get don’t do anything funny or don’t have safeguards for this.

[dberenbaum]

@efiop What do you think of the priority of this one? I haven’t seen this requested, so IMO it could be p2 or even p3.