Force using calls to package dependencies

[johnmyleswhite]

Ever since the DataFrames package had DataArrays split out, I've wanted to allow people to get both sets of functionality from a single call to using DataFrames. We currently fake this by running using DataArrays in Main when you load src/DataFrames.jl, but this is a broken hack that fails in a lot of settings.

As we start to consider what would go in the Stats meta-package, the situation gets even stranger. I imagine that Stats will want to force calls to using DataArrays, using DataFrames, using GLM, etc. It would be nice to have a convenient shorthand for doing this.

Of course, we could go through each dependency and explicitly re-export things. But I was wondering if we could potentially offer this kind of second-order dependency forcing at the language level.

[stevengj]

exportall DataArrays would be nice.

[stevengj]

Dup of #3975 and #1986.

[johnmyleswhite]

Thanks for redirecting to standing issue. Sorry for the noise.