Migrate references from mod/source plugin to core_ltix/source

• mod/lti/mod_form.php
• mod/lti/settings.php (note that settings will likely be removed as part of the admin page migration, and core will now contain the settings for managing tools.). We'll need to ensure that the ltisource settings code is present in this new settings code, as it was before in mod/lti/settings.php.
• mod/lti/servicelib (already addressed in ltix/classes/local/service/service_helper.php)
• any others that grep shows (but not backup and restore since there's a separate issue for that)