Allow using Userdev mappings more easily

[Fuyukai]

Some Forge versions (and by some versions, I mean 1.7.10) have a -userdev jar (it's the same link on forge maven, but with -userdev.jar instead of -universal.jar) which contains their own set of MCP mappings. The 1.7.10 modded ecosystem (perhaps inadvertenly, given how these are sort of half-complete) with RetroFuturaGradle uses these extensively. It's not a problem with proper remapping support except for Mixin accessors, which sometimes use the original Userdev mapping field name.

Unfortunately, neither mappings("net.minecraftforge:forge:1.7.10-10.13.4.1614-1.7.10:userdev") or mappings(volde.layered { importBaseZip("net.minecraftforge:forge:1.7.10-10.13.4.1614-1.7.10:userdev") }) works, but you can specify the path to the
CSV files manually if you've ever ran a RetroFuturaGradle import and it works fine.