RepoTree: get rid if implicit fetching

[efiop]

When we collect cache for pull/fetch or run status -c, we implicitly try to fetch the dir cache entry from the remote. And that's the behaviour that is expected from RepoTree too. There have been multiple times where we've tried to use Repo.repo_tree just to realise that it tries to fetch automatically, which is often not needed.

This PR is more of a POC that tries to get rid of fetch flag and the implicit fetching during walk() that it implies in favor of just fetching stuff explicitly where we really need it (repo has fetch method just for that already).

Need to check performance and also 2 webdav tests fail now because they don't have open() support, and Tree doesn't fetch things automatically, as expected by the api code.

Depends on #5307

• ❗ I have followed the Contributing to DVC checklist.

• 📖 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. 🙏

[efiop]

Strictly speaking this is not really needed, as cache.save() will fetch everything anyway, but I've added this for now because logic around this (e.g. tests) expects specific exceptions and also because this might be faster than save(), as we didn't optimize it yet. Need to check perf first, maybe we can remove it right now.

[efiop]

This is temporary, we'll start computing hash on-the-fly in the followup PRs.