api: fallback to external_repo if not a DVC repository

[skshetry]

Fixes #3221. Now, it will just fallback to the external_repo if Repo() could not be instantiated.
Another idea would have been to check for the existence of .dvc directory, but, I went with this, as it feels correct and robust.

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[skshetry]

open_by_relpath on the Repo also takes more arguments such as remote, etc which does not make sense here.

[Suor]

If it works it should stay as is. Let's not add ignored arguments, this will lead to silent errors.

[skshetry]

It does not as it also gets unexpected arguments from Repo.open_by_relpath:
https://github.com/iterative/dvc/blob/49039968f74282bdab3aa05b4582e5d392fc0ccc/dvc/repo/__init__.py#L430

[skshetry]

We could though add remote, or not even send one, but, I prefer this.

[Suor]

I don't understand. This line is never called from Repo.open_by_relpath().

[skshetry]

It's called by dvc.api.open() here with following args:
https://github.com/iterative/dvc/blob/49039968f74282bdab3aa05b4582e5d392fc0ccc/dvc/api.py#L70-L80

And, successively, _open calls this line:
https://github.com/iterative/dvc/blob/49039968f74282bdab3aa05b4582e5d392fc0ccc/dvc/api.py#L94-L100

[Suor]

Ok, then we should add only remote param not **kwargs and assert remote is None. This way things won't be silently ignored.

[efiop]

@Suor assert is not needed, as the point of that method is to provide the same interface for git and dvc repos, so it should be silently ignored there. Or am I missing something?

[skshetry]

Third argument is buffering.

[efiop]

@skshetry Please check the tests, they've failed.

[codecov]

Codecov Report

Merging #3222 into master will decrease coverage by 0.33%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3222      +/-   ##
==========================================
- Coverage   91.67%   91.33%   -0.34%     
==========================================
  Files         139      139              
  Lines        8681     8681              
==========================================
- Hits         7958     7929      -29     
- Misses        723      752      +29

Impacted Files	Coverage Δ
dvc/scm/git/__init__.py	88.39% <ø> (ø)	⬆️
dvc/system.py	70.17% <0%> (-17.55%)	⬇️
dvc/analytics.py	93.15% <0%> (-2.74%)	⬇️
dvc/daemon.py	54.23% <0%> (-1.7%)	⬇️
dvc/external_repo.py	88.61% <0%> (-1.63%)	⬇️
dvc/utils/fs.py	96.15% <0%> (-0.77%)	⬇️
dvc/utils/__init__.py	92.59% <0%> (-0.62%)	⬇️
dvc/stage.py	96.27% <0%> (-0.38%)	⬇️

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[Suor]

If it works it should stay as is. Let's not add ignored arguments, this will lead to silent errors.