Handle SparseArrays via Pkg extention?

[oschulz]

Can we turn SparseArrays into a weak dependency? It adds quite a bit to the load time of Distributions. From what I can see, we don't use it in methods that don't also have a sparse array as an argument, right?

[devmotion]

SparseArrays is only a test dependency, so this is rather a task for the dependencies of Distributions. I'm not sure about other dependencies, but for PDMats there is already an open PR: JuliaStats/PDMats.jl#188

[oschulz]

Sorry @devmotion , I misread my @time_imports output, overlooked an indent. Sorry for the noise.