accesslib: Move check_enrolment_plugins() to complete_user_login()

accesslib: Move check_enrolment_plugins() to complete_user_login()

... where it belongs ;-)

load_all_capabiloties() gets called at several points where we don't
want to be re-querying the enrolment backends. It needs to  be called
before load_all_capabilities() and only by callers that are setting up
a logon session.

Those callers need to be calling complete_user_login() anyway, as they
need to set the Moodle cookie, log the logon action, etc. In fact,
those callers duplicate a lot of that code already.

The callers that don't duplicate code for the login are actually the
cases where the backend enrolment plugins should not be queried.

To be followed by callers cleanup...