We can make address a lot of dependency issues if we move the Source classes to meas_base, because we'll no longer have low-level code (e.g. afw::table persistence) in the same module as very high-level code (e.g. slots). It will also put all the slot code in the same place, instead of spreading it across two pacakges.

This should be straightforward, except that we'll have a lot of downstream code to (trivially) change, and there's a good chance Swig will get confused somewhere along the way.