depinst.py ignore flag

How does the --ignore flag work in `depinst.py`?

Example:

```
git clone https://github.com/boostorg/boost
cd boost
git submodule update --init tools/boostdep
git submodule update --init tools/quickbook
python3 tools/boostdep/depinst/depinst.py --ignore unordered ../tools/quickbook
```

Notice `--ignore unordered`

The verbose output of `depinst.py -vv` shows:

`-N: ['unordered']`

Then, would it be expected if the submodule `unordered` is downloaded/cloned? It is.