Make lazy tokens lazier

[SquidDev]

• Defer reading the user-supplied classpath until needed.
• Pass around FileCollections instead of Set<File>s, so we don't need to resolve the contents until actually needed.

[marchermans]

I support this fully. But for documentation reasons: why is this change needed?; Other then to comply with gradles examples that is....

[SquidDev]

I've been fiddling with a couple of alternative approaches to #924, as from conversations on Discord, people weren't very happy with that PR.

I ended up having a few issues where we end up trying to evaluate tokens when configuring a task, which causes deobf dependencies to be fetched, which in turn causes all sorts of other problems, and so it felt worth fixing that first before going any further.

[SquidDev]

Thanks!