Use new event_manifest.yml file to generate edition files

[magnusbaeck]

Description

Now that eiffel-community/eiffel#353 has been merged, the protocol repository contains a YAML file that describes the available editions and what event types and versions exist in that edition. We can use that to simplify
internal/cmd/editiongen/main.go.

Motivation

Less code to maintain.

Exemplification

N/A

Benefits

The end result should be identical to what we have now so in that sense it won't make a difference, but if we can do the same thing with less code that's a good thing.

Possible Drawbacks

We'll introduce a dependency to a new metadata file in the protocol git. On the other hand we get rid of a dependency to the protocol git's directory layout (and that it needs to be a Git repository).