Long term re-org

[darsnack]

Ref #48 (comment)

1. Rename LearnBase.jl ➡️ LearnAPI.jl (similar to StatsAPI.jl and DataAPI.jl)
2. Create MLBase.jl as an umbrella over MLDataPattern.jl, MLLabelUtils.jl, LossFunctions.jl, and PenaltyFunctions.jl

For (2), do we want an umbrella package or consolidation of code? Right now, I prefer the former to maintain small dependencies for people who need them. But maybe after those packages get cleaned up, they will be trivially small.