Sidebar has issues pass second level

So I've tried to make a sidebar on a project that has more then a single level (i.e. folder structure as `project/nbs/one/two`) and experiencing issue where the second level files are not grouped together, as shown here:
```
sidebar:
contents:
- index.ipynb
- 00_core.ipynb
- section: a
contents:
- a/one.ipynb
- a/two.ipynb
- section: b
contents:
- a/b/four.ipynb
- section: a
contents:
- section: b
contents:
- a/b/three.ipynb
```
The older version (2.0.7) works ok, and produces correct result:
```
sidebar:
contents:
- index.ipynb
- 00_core.ipynb
- README.md
- section: a
contents:
- a/one.ipynb
- a/two.ipynb
- section: b
contents:
- a/b/four.ipynb
- a/b/three.ipynb
```

Looks like [this](https://github.com/fastai/nbdev/pull/927/files) PR solved the issue https://github.com/fastai/nbdev/issues/922 but introduced this one.