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/migrate.go

@@ -14,12 +14,13 @@ import (
 	"github.com/urfave/cli/v3"
 )
 
-// CmdMigrate represents the available migrate sub-command.
-var CmdMigrate = &cli.Command{
-	Name:        "migrate",
-	Usage:       "Migrate the database",
-	Description: `This is a command for migrating the database, so that you can run "gitea admin create user" before starting the server.`,
-	Action:      runMigrate,
+func newMigrateCommand() *cli.Command {
+	return &cli.Command{
+		Name:        "migrate",
+		Usage:       "Migrate the database",
+		Description: `This is a command for migrating the database, so that you can run "gitea admin create user" before starting the server.`,
+		Action:      runMigrate,
+	}
 }
 
 func runMigrate(ctx context.Context, c *cli.Command) error {