api: get_url does not support links for granular file 

`get_url` does not support granular files access.

Eg:
```sh
mkdir dir
echo "foo" > dir/file

dvc add foo
python -c '
import dvc.api
dvc.api.get_url("dir/file"
'
```

throws following exception:
```
OutputNotFoundError: unable to find DVC-file with output 'dir/file'
```

This also affects #3156 which introduces `dvc get --show-url`.