core: introduce "backends" to replace "remote state" (superset) and fix UX by mitchellh ยท Pull Request #11286 ยท hashicorp/terraform

๐Ÿ˜ƒ ๐Ÿ‘‹ This PR introduces "backends" as a concept to replace "remote state" (and support more features now and in the future). In the process, it ended up smoothing over a lot of rough edges and intr...