Use default branch slug on file paths without it

Problem/Motivation

If we want to cross-link file links, we'd need a predictable way of doing it.

Currently we have:
- https://api.drupal.org/api/drupal/core%21themes%21starterkit_theme%21REA...
- https://api.drupal.org/api/drupal/README.md/11.x

And the 11.x is required.

Proposed resolution

It'd be great if we can omit the branch slug, and then, when the URL matches that of a path (contains a dot???), append the slug of the default branch for that project.