This is a rather-messy, complex change to get the "command" package
building again against the new backend API that was updated for
the new configuration loader.
A lot of this is mechanical rewriting to the new API, but
meta_config.go and meta_backend.go in particular saw some major
changes to interface with the new loader APIs and to deal with
the change in order of steps in the backend API.
This helper is a Predictor for the "complete" package that tries to
auto-complete workspace names from the current backend, if it's
initialized and operable.
The predictors built in to the "complete" package assume that the same
type of argument is repeated indefinitely, but most Terraform commands
don't work like that, so this helper allows us to define a sequence of
predictors that apply to each argument in turn.
Martin Atkins