external_repo: use RepoTree for fetch/import/get

[pmrowla]

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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. If the CLI API is changed, I have updated tab completion scripts.

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Related to #3811.
Will fix #3611.

Changes:

• erepo: use GitTree instead of git checkout for cloned repos (unless dvcx for_write=True)
• erepo: use RepoTree/cache.save() for fetching external DVC+ git files
• erepo: remove pull_to
  • RepoDependency: use erepo.fetch_external() -> cache.checkout with host repo cache
  • get: use erepo.fetch_external() -> cache.checkout() with erepo tmpdir cache
• ls: use RepoTree

[efiop]

This is great!

For the record: discussed next steps to possibly unify and abstract trees by, possibly, splitting BaseRemote into one Remote + S3Tree(and so on for each type) + Cache.

[pared]

LGTM