Sorry, it still doesn't work. .get return an empty string if no match found, and it fails:

from pathlib import Path
Path().absolute().relative_to('')
# ValueError: '/home/gram' does not start with ''

Please, include a few test cases. This should help to see what is possible:
https://github.com/dephell/dephell_discover/blob/master/tests/test_root.py#L42

Thank you for taking care of this. I've done with most of my other projects, so now I'll try to give you feedback faster :)

I think I figured out the reason for that. I set the default value to "" (empty) instead of the current directory ".".
I rewrote the code to check if the dictionary is empty and if not, it takes the first value of the dictionary. This is usually only one entry.

I must confess, I don't know where I can write those testcases. You wrote all testcases very generic in the test_all.py file. All other converters only implement testcases for loading.
I try to stick to your scheme of implementing those tests, but at the moment, it is a little bit overwhelming.
Could you give me a hint or an example how to implement the dump test for the wheel package?

Taken from your next comment:

"Using the current dir as default is not correct."

If the local folder is not the best default value for that, what would you consider a good alternative?
I think, if the project folder was set for another path, that path should be in that structure nevertheless.

Falling back to the value project.package.path might be a safe choice for that situation.

Figured out another approach, which corresponds with how setuptools is generating the wheel package.
The exact approach is described in the commit messages and in the source code itself.