explicit dependencies in setup.py, explicit zcml loading in tests.

removes dependency on Plone and adds dependencies to all imported packages.
The only place plone.api (without testing) needs Products.CMFPlone is in https://github.com/plone/plone.api/blob/cleanup-dependencies/src/plone/api/user.py#L5 
Depedning on ``Plone`` package is imo a bad idea.