Fixes#12871
We were forgetting to remove the legacy remote state from the actual
state value when migrating. This only causes an issue when saving a plan
since the plan contains the state itself and causes an error where both
a backend + legacy state exist.
If saved plans aren't used this causes no noticable issue.
Due to buggy upgrades already existing in the wild, I also added code to
clear the remote section if it exists in a standard unchanged backend
Fixes#11504
The local backend should error if `terraform plan` is called in a
directory with no Terraform config files (same behavior as 0.8.x).
**New behavior:** We now allow `terraform plan -destroy` with no
configuration files since that seems reasonable.
The local backend implementation is an implementation of
backend.Enhanced that recreates all the behavior of the CLI but through
the backend interface.
Mitchell Hashimoto