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>

tests/integration/cmd_keys_test.go

@@ -10,7 +10,6 @@ import (
 
 	"code.gitea.io/gitea/cmd"
 	"code.gitea.io/gitea/modules/setting"
-	"code.gitea.io/gitea/modules/test"
 	"code.gitea.io/gitea/modules/util"
 
 	"github.com/stretchr/testify/assert"
@@ -38,13 +37,14 @@ func Test_CmdKeys(t *testing.T) {
 		for _, tt := range tests {
 			t.Run(tt.name, func(t *testing.T) {
 				// FIXME: this test is not quite right. Each "command run" always re-initializes settings
-				defer test.MockVariableValue(&cmd.CmdKeys.Before, nil)() // don't re-initialize logger during the test
+				keysCmd := cmd.NewKeysCommand()
+				keysCmd.Before = nil // don't re-initialize logger during the test
 
 				var stdout, stderr bytes.Buffer
 				app := &cli.Command{
 					Writer:    &stdout,
 					ErrWriter: &stderr,
-					Commands:  []*cli.Command{cmd.CmdKeys},
+					Commands:  []*cli.Command{keysCmd},
 				}
 				err := app.Run(t.Context(), append([]string{"prog"}, tt.args...))
 				if tt.wantErr {