Deprecate support for local plugin navigation callback without the local_ prefix.

Presently we have the following two navigation callbacks:

function local_nicehack_extends_navigation(global_navigation $nav);
function local_nicehack_extends_settings_navigation(settings_navigation $nav, context $context);

We also support mapping the following to the first callback above:

function nicehack_extends_navigation(global_navigation $nav);

After the release of 2.3 I think it is in our best interests to add a debugging call to the old alternative so that in 2.4 people get advised to upgrade and in 2.5 we can remove it.