The new config loader requires some steps to happen in a different
order, particularly in regard to knowing the schema in order to
decode the configuration.
Here we lean directly on the configschema package, rather than
on helper/schema.Backend as before, because it's generally
sufficient for our needs here and this prepares us for the
helper/schema package later moving out into its own repository
to seed a "plugin SDK".
What will hopfully be the final version of the Backend interface. This
combines the MultiState interface into Backend since it will be required
to implement, and simplifies the interface because the Backend is no
longer responsible for tracking the current state.
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.
Martin Atkins