Rework how MixinConstraints disable mixins.

[Fox2Code]

Allows to only call MixinConstraintsBootstrap.init(); once and have MixinConstraints work everywhere.

[Moulberry]

shouldApplyMixin is called before the target class is attempted to be loaded.
This change causes the checks to happen after the target class is attempted to be loaded.

This results in the log being filled with ClassNotFoundErrors when using IfModLoaded annotations on a mod's class even when that mod isn't loaded.

See 7216691 for an example case.
Doing ./gradlew :testmod:fabricRunClient on this branch with the above commit causes the following log message to appear:
[main/WARN]: Error loading class: com/doesnt/exist/Main (java.lang.ClassNotFoundException: com/doesnt/exist/Main)

I am not sure if there's a workaround to get this approach to work without spamming errors in console.

Edit: the error originates from org.spongepowered.asm.mixin.transformer.ClassInfo#forName, not MixinConstraints, in case that wasn't clear.