Fixes#12174
You're allowed to refresh with a nil module (no configs) as long as you
have state. However, if `-input=true` (default) then this would crash
since the input attempts to read the configs.
The API contract with `terraform.Context` says that the module tree must
be non-nil and loaded. To do this for other commands we create an empty
module tree. We do that here now.
The local backend implementation is an implementation of
backend.Enhanced that recreates all the behavior of the CLI but through
the backend interface.
James Bardin