Add a game version mod candidate finder

[sschr15]

This is the simplest solution to https://github.com/QuiltMC/quilt-loader/issues/15, but we can always make it more complex!

...or something else if others feel like it

[AlexIIL]

Hey, thanks for making this! However at the moment I'd rather gauge interest in this feature, rather than actually implement it directly - in particular this implementation works at the moment, but might not if/when subfolders are allowed - so more complex logic might need to be added if/when #38 is wanted.

(As a sidenote: I've had a go at implementing this in the loader-plugins-2 branch, but this feature probably wouldn't wait until plugins to actually be implemented)

[AlexIIL]

Implemented in 0.18.1-beta.1 with loader-plugins