User modules doesn't have a regular path

[ferraith]

setuptools-pyecore handles user models as regular files with a valid windows or posix paths. This assumption is wrong. From pyecoregen doc:

If specified, the given string is interpreted as a dotted Python module path. E.g. --user-module my.custom_mod will make the generated code import mixin classes from a module my.custom_mod.

To fix this problem user modules passed on the cli or defined in a setup.cfg file shouldn't be handled anymore as paths. A simple string is sufficient.