How do you eager load or join reverse relationships for entries?

Ok so here's how I ended up doing this. Essentially querying everything and adding a sourceId and targetId property to each result. In order to get this query to work though, we need to add a behavior to the Entry component to support two new properties, sourceId and targetId.

// This basically left-joins the the relationship table to our sections. This allows us
// to essentially "eager-load" in reverse.
$relatedQuery = Entry::find()
  ->section(['healthTopics', 'providers', 'locations'])
  ->addSelect('relations.targetId')
   ->addSelect('relations.sourceId')
   // You can query to get the field ids here, I just decided to look them up in the db.
    ->leftJoin(
               '{{%relations}} relations' ,
                '([[elements.id]] = [[relations.sourceId]]) AND ([[relations.fieldId]] = 2 OR [[relations.fieldId]] = 5)'
            )
    ->groupBy('elements.id')
    ->all();

    // Same thing here, just looked up the section ids instead of querying for them.
    $healthTopics = ArrayHelper::where($relatedQuery, 'sectionId', '1');
    $providers = ArrayHelper::where($relatedQuery, 'sectionId', '3');
    $locations = ArrayHelper::where($relatedQuery, 'sectionId', '2');

Next, I needed to add a behavior to the Entry component so that it could support having the two new properties we added, sourceId and targetId.

<?php

namespace foobehaviors;

use yiibaseBehavior;

class EntryBehavior extends Behavior
{
    public $targetId;
    public $sourceId;
}

Event::on(Entry::class, Entry::EVENT_DEFINE_BEHAVIORS, static function (DefineBehaviorsEvent $event) {
    $event->behaviors['entryBehavior'] = ['class' => EntryBehavior::class];
});