parsing: use relative paths; force absolute paths to relative

[skshetry]

parsing.Context now only expects the path to always be relative path. To clarify, it's relative if it's a LocalFileSystem and relatively-absolute (relative to the root of the repo) in GitFileSystem, as opposed to being absolutely-absolute paths.

This expectation for Context to always get a relative path is managed in DataResolver for now, as changing anything in higher level may not be trivial.

This change, that it uses the relative path in LocalFileSystem and relatively-absolute paths in GitFileSystem does affect dvc.params.diff(). Before we were using same relative/absolute paths, which worked for both filesystems, but now we will receive the data in something like following format:

{
    "workspace": {
        "data": {
            "../test_params.yaml": {  # relative path from LocalFS
                "data": {
                    "vars.model1.epoch": 20,
                    "vars.model2.epoch": 35
                }
            }
        }
    },
    "HEAD": {
        "data": {
            "test_params.yaml": {  # From GitFS as it is relative from the root of the repo
                "data": {
                    "vars.model1.epoch": 15,
                    "vars.model2.epoch": 35
                }
            }
        }
    }
}

So, as a result of this, this had to be transformed such that they return same paths, so that we can compare them easily.

Fix #7307.

[efiop]

@skshetry Any thoughts on how this would behave itself with moving gitfs to root paths? Asking in case you've looked into that 🙂

[skshetry]

@skshetry Any thoughts on how this would behave itself with moving gitfs to root paths? Asking in case you've looked into that 🙂

It's hard to say for all stage loading helpers, but this is how it already works with root paths of gitfs. The following code is trying to transform filesystem-absolute paths into root paths in the case of GitFileSystem:
https://github.com/iterative/dvc/blob/5d4d8585744bbe8163a32c9f78c9bb681b4eba56/dvc/parsing/__init__.py#L142-L147

[efiop]

@skshetry Great! 🔥 With this and repofs, we can now try to use treeverse/scmrepo#56