RetroFuturaGradle support

[tr7zw]

Hey there, I'm currently trying to get the source code from 1.20.4-1.16.5 using Archloom down to 1.12/1.8(/1.7?) using RetroFuturaGradle. Currently, I'm stuck at this step, as preprocessor doesn't seem able to find the mappings used by RetroFuturaGradle

IllegalStateException: Failed to find mappings from project ':1.16.5-forge' to project ':1.12.2-forge'.

Maybe it's possible to get it working by just creating fake createMcpToSrg etc tasks that return the files required by preprocessor, not sure.