Better name for jsonstatus package

internal/ui/jsonstatus and termstatus sound similar but are not related
in any way. Instead `internal/ui/backup` and `internal/ui/jsonstatus/status`
are the counterparts. Rename the latter to `internal/ui/json/backup` to
make this clear.

internal/ui/json/backup.go

@@ -1,4 +1,4 @@
-package jsonstatus
+package json
 
 import (
 	"bytes"