Use default_section instead of FIRSTPARTY as fallback for existing paths

If you `pip install -e` a requirement it is considered to be first-party
always.

This makes sense for the typical `pip install -e .`, but not when
installing requirements editable through Git, where it appears to be
useful in case you want to hack on it.

I've tried adding the following to `place_module`, but it also catches
the `pip install -e .` case.  This could then be worked around by adding
it to `known_first_party`, but then it might also be an option to have a
mode where only explicitly defined packages are considered to be
first-party?!

```
elif os.path.exists(os.path.join(prefix, os.path.basename(prefix).replace('-', '_') + '.egg-info')):
    return self.sections.THIRDPARTY
```

With this change the following config will work as expected:

    known_first_party = myproject
    default_section = THIRDPARTY