Partial revert of imoports in nipype namespace

This is a followup to my recent comment on #784. It also closes #784.

This means that you have to specifically ask to import interfaces to any external code (whether or not that code is Python-based), but the major components of nipype itself are still available in the nipype namespace.

This brings the import wall time down to 696 ms from 16.6 s on my system.