This question is similar to Support composer_manager AND drupal-composer/drupal-project with contrib module, but I am asking it with a different angle this time. Imo this justifies a separate question, as an answer to this could be generally useful beyond my specific use case.
Technical background
packages.drupal.org re-publishes module packages from drupal.org with auto-generated composer.json.
The main "module package" becomes a composer package with "type": "drupal-module". Each "submodule" (module in a subdirectory) becomes a package with "type": "metapackage".
The composer.json for each submodule is generated from the *.info file. E.g. if the *.info file says dependencies[] = composer_manager, then the composer.json for this metapackage will require drupal/composer_manager. It seems that composer.json in the submodule is ignored.
For the main / root package, perhaps an existing composer.json is not ignored. Not sure about that.
Sometimes the generated composer.json is not exactly what I want.
The functionality on packages.drupal.org is provided by https://www.drupal.org/project/project_composer. There are database tables that contain information about modules uploaded to drupal.org (via https://www.drupal.org/project/project), and project_composer derives Composer data from this.
Question
Is it possible for a contrib module to alter the composer.json that will be generated for the main module package and for a submodule metapackage?
E.g.:
- Prevent a module dependency from the *.info file to be added as a Composer requirement.
- Add an additional 3rd party Composer requirement.
