Extract pipeline to be usable in SpongeVanilla's installer

[zml2008]

VanillaGradle and the SpongeVanilla installer have to perform a lot of the same steps to produce a Minecraft server jar, and currently duplicate quite a lot (downloading, hash validation, version manifest handling, etc). VanillaGradle has also had a fair bit of work go into improving the remapping output and providing an efficient downloader, which would be nice to have available on the installer without having to copy & paste.

The current VanillaGradle pipeline is already fairly isolated from Gradle -- it should only need some basic restructuring in order to be flexible enough to handle the needs of the installer (resolve a versions.json, skip stripping shaded dependencies, no pre-baking of AWs).