General: Do not @include modules

In 3016571-t, a user reported that his site went down upon upgrading to 4.5. After some debugging by the user, it was found that there was a conflict between 4.5 and another plugin where an `Upcoming_Events_Widget` class was being defined twice.

This should've been easy to figure out, except I was unable to find any error output. Even with `WP_DEBUG` and `WP_DEBUG_LOG` set to `true`, there was no error output on the front of the site, in `wp-content/debug.log`, or in the system's error logs. 😱

I believe this adds unnecessary frustration for our users and HEs when debugging and we should find a way around the `@include`.

The `@include` was introduced in #3334.

Review the @include introduced in issue #3334 and trace how duplicate class errors are currently loaded and reported. Reproduce the upgrade conflict described here, then verify that the resulting error is visible on the site or in the configured debug and system logs without suppressing it.

Written by the indexing model from the issue text.