add legacy fabric support

[thecatcore]

superseeds #5

[jamierocks]

This passes off third-party intermediaries as being Fabric's - they should have unique uid.

[thecatcore]

Doesn't FabricLoader depend on "net.fabricmc.intermediary" component to run? Changing the uid of LegacyFabric intermediary will require to change this too in this case 🤔.

[thecatcore]

I tried to keep them separated at least in the upstream-meta repo, but not sure how to do it for the multimc-meta one

[jamierocks]

I would not expect MultiMC to pass this off as Fabric anyway, it's a third-party project.

Doesn't FabricLoader depend on "net.fabricmc.intermediary" component to run?

Yes it does.

Changing the uid of LegacyFabric intermediary will require to change this too in this case

No it shouldn't. I can think of two solutions to this off the top of my head:

1. Have a Legacy Fabric 'Fabric Loader' component, which depends on it's own intermediaries.
2. Remove the requirement from the Fabric Loader component.

[thecatcore]

Updated accordingly

[thecatcore]

How do I add "noapplet" trait to legacy fabric variant of Loader?

[thecatcore]

Ready for review