dashed derivation names

[t184256]

Python does not allow dashes in module names, while some nix derivations use them in names (ex: setuptools-git).

We can either:

1. try to resolve a dashed name given an underscored one, preferrably on the error path
2. devise a hacky convention to work around than, like '__' -> '-'
3. devise an alternative import syntax free from that limitation