Switch cmd/ to use constructor functions. (#36962)

This is a step towards potentially splitting command groups into their
own folders to clean up `cmd/` as one folder for all cli commands.
Returning fresh command instances will also aid in adding tests as you
don't need to concern yourself with the whole command tree being one
mutable variable.

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

cmd/doctor_convert.go

@@ -14,12 +14,13 @@ import (
 	"github.com/urfave/cli/v3"
 )
 
-// cmdDoctorConvert represents the available convert sub-command.
-var cmdDoctorConvert = &cli.Command{
-	Name:        "convert",
-	Usage:       "Convert the database",
-	Description: "A command to convert an existing MySQL database from utf8 to utf8mb4 or MSSQL database from varchar to nvarchar",
-	Action:      runDoctorConvert,
+func newDoctorConvertCommand() *cli.Command {
+	return &cli.Command{
+		Name:        "convert",
+		Usage:       "Convert the database",
+		Description: "A command to convert an existing MySQL database from utf8 to utf8mb4 or MSSQL database from varchar to nvarchar",
+		Action:      runDoctorConvert,
+	}
 }
 
 func runDoctorConvert(ctx context.Context, cmd *cli.Command) error {