Fix: changes to ATs break multi-module projects

[LakMoore]

I'm working on a multi-module Minecraft Forge project. We were originally using a build script that compiled the dependency-modules into Jars and then had the master module depend on the Jars. I upgraded the buildscript to work as a true Gradle Multi-module project but, since then, when pulling commits that included changes to AccessTransformers, IDEA would complain about not being able to find the sources.

I think I have stumbled on the solution and would be very pleased and grateful if you could merge this PR.

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Quitting early if AT_HASH does not match athash removes the chance for us to generate a new artifact in some scenarios when the ATs have been changed.

Making the comparison here in MinecraftUserRep::findFile is problematic and not necessary. The HashStore will look for a cache hit/miss in later find*** calls. The HashStore compares the current AT_HASH with the cached AT_HASH and will return a Cache Miss if the ATs have changed.