Clarification of the neccessary autoloading of plugins...

...in the error message

Added links to the relevant documentation
I like this structure much better. 

I would suggest changing to be of the form:

    <?php
    if (empty($plugin)) {
        return;
    }

    if (!Plugin::loaded($plugin))) {
        // existing message
    } else {
        // composer message
    }

That makes the help specific to the scenario and means "If this belongs to a plugin" can be removed.

Since the element would not then be composer specific perhaps just name it plugin_class_warning or similar.