`dvc add` command incorrectly raises a `DoesNotExistError` when a
broken symlink exists in an output directory, and the target name
is same as the directory's name.
eg: If `data` is an output, and is the command is invoked as `dvc add data`
(i.e. no virtual directory operations to perform).
The expected behavior to raise a `FileNotFoundError`.
`DoesNotExistError` should only be raised if the output itself does not
exist.
Related: https://github.com/iterative/dvc/issues/3717
skshetry
5379aa2348