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.
Backends are a mechanism that allow abstracting the behavior of
Terraform CLI from the actual core. This allows us to slip in special
behavior such as state loading, remote operations, etc.
James Bardin