[1.20.1] Mod incompatibility with Fusion causing crash with Create and potentially others

[karelmikie3]

First of all since this is a prerelease I understand that you don't have time to sort mod incompatibilities so it is fine if this doesn't get fixed right away. However, I have discovered an incompatibility with Fusion which causes a crash when combined with Create.

Mod versions:
PAUCAL: 0.6.0-pre-118
Hex: 0.11.1-7-pre-620
Caelus API: 3.1.0+1.20
Patchouli: 1.20.1-84
Create: 0.5.1.f
Fusion: 1.1.0c
Forge: 47.2.0

Here are the logs when all mods are used:
https://gist.github.com/karelmikie3/a9c4af832fa95070b20237c2ac87d341

Now for my solution and commentary:
Looking at the log it's easy to point out ForgeMixinDatagenModLoader. Looking at its code shows an overwrite mixin which could be incompatible with a similar mixin in Fusion. Changing it into an inject as done in #621 alleviates this issue.

I however, have some doubts about the necessity of this mixin entirely as looking at forge's code suggests that this code would behave identically with and without the mixin.