[7.0] Dependency Rework

[Jonathing]

This PR is, what I hope to be, the final large rework to ForgeGradle 7 before it is released from Beta. It's still a work-in-progress and I hope to finish tomorrow. Here's some of what it does:

• [X] Use attribute disambiguation rules instead of dependency subtitution for OS, mappings channel, and mappings version attributes.
• [ ] Completely rework how Minecraft dependencies are handled and resolved.
• [ ] Add syncMavenizer tasks for all source sets (even if they do not have any mavenizer tasks) that are called before the source set's compile and Slime Launcher tasks. Fixes #1013.
• [ ] Makes the code less shitty.

Once this PR is finished and approved, I will immediately merge #1001 if there are no outstanding issues. If after a few hours (or days, depending) there are no further outstanding issues with ForgeGradle 7, 7.0.0 will be released and I will be free (:clueless:).