MCPTask should return a JSON that exposes key files

[Jonathing]

Currently, the Forge buildscript (yes, the buildscript, not ForgeDev) uses some Download tasks that don't do any caching (and when they do, it's inconsistent). The vast majority of tasks download files that Mavenizer already either downloads or processes itself. It would be much better if in MCPTask, an output JSON (defined by an argument) would show key locations such as client raw, version json, etc.