Constants should always be only assigned once. The logical OR assignment
of a constant is a relic of the past before we used zeitwerk for
autoloading and had bugs where a file could be loaded twice resulting in
constant redefinition warnings.
This happened when a middleware accessed the `currentUser` before a controller had a chance to populate the `action_dispatch.request.path_parameters` env variable. In that case Discourse would always cache `nil` as `currentUser`.
This consolidates logic used to match routes in ApiKey, UserApiKey and DefaultCurrentUserProvider. This reduces duplicated logic, and will allow UserApiKeysScope to easily re-use the parameter matching logic from ApiKeyScope
