Yii2 module (not under /vendor folder) class not found in config/main.php

I've created a module under folder api (which itself is same level as backend & frontend in yii2 advanced app).

Folder structure:

api
-- common
------ controllers
------ models
-- config
-- modules
------ v1
---------- controllers
---------- models
-- runtime
-- tests
-- web

In api/config/main.php:

return [
'id' => 'app-api',
'basePath' => dirname(__DIR__),    
'bootstrap' => ['log'],
'modules' => [
    'v1' => [
        'basePath' => '@app/modules/v1',
        'class' => 'api\modules\v1\Module'
    ]
],

I get error:

ReflectionException

Class api\modules\v1\Module does not exist

The class is definitely there.

I've run a composer dumpautoload (no errors).

urlManager in config/main.php:

'urlManager' => [
        'enablePrettyUrl' => true,
        'enableStrictParsing' => true,
        'showScriptName' => false,
        'rules' => [
              'v1/site/index' => 'v1/site/index', //module/controller/action
        ],        
    ]

Any help much appreciated,

thanks