dvc: get rid of path_info

Intermediate PR in removing path_info from our code base.

It is completely removed from the main codebase and is still
being used in tests, which will be migrated in the followups.

For fs-specific simple path manipulation, we've introduced
`fs.path` helper (similar to `os.path`).

Fixes #6774

tests/unit/repo/test_repo.py

@@ -28,7 +28,7 @@ def test_find_outs_by_path(tmp_dir, dvc, path):
 
     outs = dvc.find_outs_by_path(path, strict=False)
     assert len(outs) == 1
-    assert outs[0].path_info == stage.outs[0].path_info
+    assert outs[0].fs_path == stage.outs[0].fs_path
 
 
 def test_find_outs_by_path_does_graph_checks(tmp_dir, dvc):