fetch: introduce --tree-only

[efiop]

Very useful plumbing command to be able to restore full data index before actually downloading files.

This should be used as a step in regular fetch in the future instead of implicit cloud.pull() in output.get_dir_cache.

[dberenbaum]

If it's exposed in dvc fetch, what about --dir-only since it better matches the terminology on the user side with .dir files?

[efiop]

Thought about --dir-only, but that is leaking the internal object name into the outside, while the aim is to fetch the structure (trees) of your repo. --dir-only also makes sense if we go with #8884 and make our outputs use self.files first and not try to convert them into .dir object like we do right now. Taking a look at that last problem right now and might rename the flag here.

[efiop]

Closing for now, might revisit later