Make ImageSegmentation a subpackage of Images?

[johnnychen94]

Previously ImageSegmentation depends on Images, but now we're making it depending on ImageCore and ImageFiltering, so currently it's possible that we do @reexport ImageSegmentation in Images.

My vision on Images.jl is to move stable packages to Images.jl repo (https://github.com/JuliaImages/Images.jl/issues/898) and reexport them from Images.jl. And I would also like to make our centralized documentation juliaimages.org to Images-exclusive; this said, the current ImageFeatures should host its own documentation unless we make it depending on smaller dependencies and included by Images like ImageSegmentation (if we do this way).

[timholy]

but now we're making it depending on ImageCore and ImageFiltering

~Indeed even the latter will be only a test dependency. But your core point stands, of course~ nvm, it's always been a dependency...