Moving zypp_plugin to a dedicated directory

Previously zypp_plugin.py was installed inside of the root of python
modules. I'm going to add another helper module to the zypper python
plugin module, hence it makes sense to have all these files under a
dedicated directory rather than have them sparse inside of python's
modules root.

This commit updates all the references made to zypp_plugin.
Added zypper lock helper module

This module makes easier to read,write,add and remove zypper locks.
Using this module we won't need to remove the whole lock file every time
we invoke the 'setLock' capability.

Updated the Makefile: added a new 'test' task which runs lock_helper.py
tests.