Resolve bi-directional dependecies

[ronpandolfi]

As pointed out by @jklynch and @tacaswell , bi-directional dependencies exist in xi-cam.core and xi-cam.plugins. Besides making us more susceptible to circular import issues, this also make packaging with conda more difficult.

[mrakitin]

I'd like to clarify that the packaging itself with conda is not difficult in this case. It's the testing (optional, but highly preferred) bit of the packaging which is failing when attempting to import a package which tries to import a sibling package not declared as a dependency.