cliconfig: Move HCL 1.0-based language out of the way

We've been continuing to maintain the HCL 1-based CLI configuration
language to keep compatibility with prior versions, but the HCL 1 API is
constraining and it's annoying to have both versions of HCL in Terraform
at once.

This commit doesn't change any user-visible behavior but it does rename
a bunch of things in the cliconfig package to include the word "Legacy" so
that we can gradually build out an HCL 2 based new implementation
alongside and eventually support them both together.

The plan to support them both at once is to also switch over to using
OS-standard configuration file locations (like XDG Base Directory on Linux)
at the same time and treat the new locations as an opt-in for the new
language variant, with both continuing to be supported for the forseeable
future but the legacy one likely becoming increasingly more deprecated as
it ages.

checkpoint.go

@@ -18,15 +18,15 @@ var checkpointResult chan *checkpoint.CheckResponse
 
 // runCheckpoint runs a HashiCorp Checkpoint request. You can read about
 // Checkpoint here: https://github.com/hashicorp/go-checkpoint.
-func runCheckpoint(c *cliconfig.Config) {
+func runCheckpoint(c *cliconfig.LegacyConfig) {
 	// If the user doesn't want checkpoint at all, then return.
 	if c.DisableCheckpoint {
 		log.Printf("[INFO] Checkpoint disabled. Not running.")
 		checkpointResult <- nil
 		return
 	}
 
-	configDir, err := cliconfig.ConfigDir()
+	configDir, err := cliconfig.LegacyConfigDir()
 	if err != nil {
 		log.Printf("[ERR] Checkpoint setup error: %s", err)
 		checkpointResult <- nil