Split packages

In `conda-build` 3 it is possible to have multiple package outputs.

This can be useful if a build requires to compile multiple pieces of source together, but is better distributed as separate packages after the fact (e.g. LLVM, gcc, APR, etc.).

However another use case is distributing multiple components of a package with different contents (e.g. devel, debug, so, src, docs, etc.). This also came up in issue ( https://github.com/conda/conda-build/issues/2363 ). This would be useful for a lot of packages where users want extra features (e.g. debug), but also want to have a lean version for Docker images and other deployments (e.g. so). It's also possible to generate a legacy metapackage, which pulls all of the contents back together.

It would be good to figure out what sort of split packages we might want to generate in conda-forge. Also would be good to discuss how we want to integrate their use into other downstream conda-forge packages. Would also be worth thinking about whether there will be any effect on our tooling (e.g. `conda-smithy`).

cc @conda-forge/core @epruesse