Use metadata directory instead of re-extracting every launch

[PaintNinja]

This addresses the todo comment inside SlimeLauncher's main class. Needs the complimenting FG7 PR to work: MinecraftForge/ForgeGradle#1084

[LexManos]

I think because both FG and FDP use this, and having to update both is a pain.
The better option would be to continue to support the input as a zip, but supporting a directory.
I am also not a fan of Gradle's zipTree as it extract EVERYTHING in the zip to disc twice while doing the extract for the specific files you want. So its rather slow.

But, I think the better way to address the TODO would just to skip extracting entirely. Instead just reading the file from the zip to memory using JsonData.minecraftVersion(InputStream)

I think the TODO itself, is just old, as the FG side doesn't extract the archive anymore.
I'm wrong on that