prioritize enaml over py extension only within enaml.imports() context:

[AndiEcker]

with these little changes we can have enaml and py files within the same project or folder.

So e.g. having the files app.py and app.enaml the code::

enaml.imports(): from app import Main

will import Main from app.enaml and not try to import it from app.py and then fail.

[MatthieuDartiailh]

Close based on #410 discussion