define a template file for a field item in my modules
In hook_theme you define and register your templates. 

In your code your template will be "myplugin".
To use that template you need to specify "#theme" => "myplugin" in your render array (not my-field-item-template). Then twig tries to render your content with the file called "myplugin.html.twig". This twig file should be stored in the templates directory of your module.

You may read this https://www.drupal.org/docs/8/theming/twig/debugging-twig-templates to learn more about debugging theming.