Stop re-exporting all of Distributions

[penelopeysm]

Turing.jl/src/Turing.jl

Line 7 in d8fbe78

@reexport using Distributions, MCMCChains

The same as #2681, but for Distributions.

I think the complete re-export of Distributions is almost justifiable. In particular, we definitely want to export all the distributions themselves, as well as functions like truncated, product_distribution, perhaps even logpdf.

But Distributions exports a ton of things that are not the above:

https://github.com/JuliaStats/Distributions.jl/blob/bbdd4f15f3230f42c8856f56b74770e346178f43/src/Distributions.jl#L32-L277

And I think we can afford to be more selective.