Content Pack installation does not detect existing Event Definition entities

Given a content pack containing:
- a Correlation Event Definition ("ED-2") that relies on an underlying Event Definition ("ED-1")
- the Event Definition ("ED-1") required by ED-2, and
- the custom Stream tied to all Event Definitions

If the Stream and ED-1 already exist on the system and you install this new content pack, it will create a duplicate ED-1 but not a duplicate Stream.

This indicates there is some intelligent matching of Stream entities in Content Packs to pre-existing ones on the target system, but there is no such matching for Event Definitions.

## Expected Behavior
If an Event Definition that a content pack provides already exists (by name not ID) on the target system, then content pack installation should skip the installation of its copy of the Event Definition.

## Current Behavior
Content Pack installation creates a copy of the Event Definition (with a different object ID of course).

## Possible Solution
Whatever intelligent matching is done for Streams in this fashion should be done as well for Event Definitions too.

## Steps to Reproduce (for bugs)
1. Install [this Content Pack](https://github.com/Graylog2/graylog2-server/files/14983805/Events.Course.-.Challenge.1.Content.Pack.json) first
2. Then install [this one](https://github.com/Graylog2/graylog2-server/files/14983806/Events.Course.-.Challenge.3.Content.Pack.json)

## Context
Affects my ability to install Correlation Event Definitions based on content previously built upon.

## Your Environment

* Graylog Version: 5.2.6
* Java Version: bundled w/ Graylog