push/pull: support an aggregate path as a target that has multiple outputs inside

[skshetry]

Let's say we have the following hierarchy, where data/bar and data/foo are individual outputs.
When you do dvc push data, dvc does not know to search for outputs inside the path.

.
└── data
   ├── bar
   └── foo

At the moment, target only supports filtering subpath of an output or an exact path to the output.

So, it would work for data/foo or data/bar. And, if the data was being tracked as single output, data/bar and data/foo will work to filter the data output.

---

Regarding implementation, we have internal utilities to collect all outputs of a path using find_outs_by_path(recursive=True). But since it can return multiple stages, there may be assumptions that it can return only a single output/stage which have to be fixed. And, more importantly, test that it does not affect other commands, since this collect_granular is used in a lot of places.

dvc/dvc/repo/stage.py

Line 396 in 44b78b8

(out,) = self.repo.find_outs_by_path(target, strict=False)

Related: #10203.

[dberenbaum]

@skshetry Is it only about push/pull? Does it impact checkout or other commands?

[skshetry]

Yes, it likely affects all of the commands, as most of them call collect_granular.