Fix bugs in setup.py collector

I tried to use the nose2 collector with setup.py but I found that the imports were broken. 

The collector tried to import the main module but it was importing the main function that was exported by \__init__.py instead.  Fixed by explicitly importing the right class.

Having fixed that issue, the collector tried to pass a tuple to a method that now expects a list.  I changed the method to be more forgiving.