A stab at davidhalter/jedi#361 (Flask extension imports)

Both new-style and old-style extensions work, but only when imported
with a 'from'.  There are two skipped tests of the full dotted name
imports.

Also, our fixture has a normal flaskext package, whereas in practice
the flaskext module is injected from a pth file and does not have
__init__.py, we need to figure out to handle that.
Make tests terser. pytest rules!
Fix an exception in the flask ext code.