Stronger artifact model

[zml2008]

Currently all the VanillaGradle infrastructure to resolve minecraft is hardcoded around the 3 known dependencies and the AW modifier.

This is not the only way to represent the game -- see #32, and look at patcher deps or platforms that might want to use a different system for merging jars, and we should make our system more flexible to accommodate that. The API of Artifactural provides a good reference to start from, but is probably not a good fit to use directly, since it doesn't mesh well with VG's current design, and doesn't seem to have a super strong representation of metadata (which should be a first-class artifact of its own).