In order to generate composer.json, Drupal.org defines specific rules in the modules info.yml file
If you need to add a dependency to the Drupal.org module you should provide a format:
dependencies:
- drupal:webform
In this case, your module will have composer dependency to drupal/webform
If you make it:
dependencies:
- whatevernameyouwish:webform
the Drupal.org packaging routine will replace it with drupal:webform on the fly.
In order to break the dependency on composer level but still tell Drupal core to have module dependency while resolving dependencies during the process of enabling the module, you should use the simplified format:
dependencies:
- webform
In the above case, composer won’t have any dependencies, but your module will require that the webform module be available in the codebase in order to be enabled by Drupal core.