dvc: properly handle relative symlinks

[efiop]

Fix #4160

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• 📖 If this PR requires documentation updates, I have created a separate PR (or issue, at least) in dvc.org and linked it here.

Thank you for the contribution - we'll try to review it as soon as possible. 🙏

[pared]

I wonder what about followin situation:

1. We have a link in our data dir
2. We add it
3. We update link origin

I guess one would expect its state will also be updated in our data dir, right?

[efiop]

I guess one would expect its state will also be updated in our data dir, right?

@pared Great point! Potentially we could just keep the original link, but if someone would to think "oh, I've added it to dvc, so I can delete it now" we would get a broken link. From that standpoint it is just easier to replace the link with our own link (which in this test is copy, but might be symlink/hardlink/reflink). But let's definitely keep that use case in mind, maybe someone will come ask for it.

[pared]

Yeah, I guess there is no proper solution that would satisfy all use cases and expectations.