Move BC layer to make sure it is always loaded.

The Constant file is not always loaded (I had problems being on the startpage, clicking onto the OpenID modules select-google-as-method button in the login block, which executes ajax ending up with the OpenID's Authentication api which uses the `Users_Helper_AuthenticationMethod` class which was not loaded).

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | --
| Fixed tickets | --
| License       | MIT
| Doc PR        | --