add: raise DoesNotExistError only if the output path does not exist (#10373)

`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: #3717

Author: skshetry

dvc/output.py

@@ -1365,7 +1365,7 @@ def add(  # noqa: C901
                 dry_run=not self.use_cache,
             )
         except FileNotFoundError as exc:
-            if self.fs_path == path:
+            if not self.exists:
                 raise self.DoesNotExistError(self) from exc
             if not self.is_dir_checksum:
                 raise