support split environment mappings sets

[SpaceWalkerRS]

Loom already supports split environment source sets, but this does not extend to mappings. This is not an issue for modern Minecraft, since the client and server can be merged, and a single mappings set can apply to either or both environments. But versions prior to 1.3 cannot be merged, and thus each environment needs its own mappings set.

What I'm hoping for:

• An option analogous to the split environment source sets option, but that applies to mappings.
• A way to define intermediary/named mappings providers for each environment.