The new data source planning logic no longer needs a separate action,
and the apply status can be determined from whether the After value is
complete or not.
The state was not being set, so the change was not evaluated correctly
for dependant resources.
Remove use of cty.NilVal in readDataSource, only one place was using it,
so the code could just be moved out.
Fix a bunch of places where warnings would be lost.
The logic for refresh, plan and apply are all subtly different, so
rather than trying to manage that complex flow through a giant 300 line
method, break it up somewhat into 3 different types that can share the
types and a few helpers.
