reexport as submodule

[johnnychen94]

I'm not sure if this is doable:

module GiantPkg

    @reexport A using SubPkgA
end

that mimics

module GiantPkg

    module A
        using Reexport
        @reexport using SubPkgA
    end

end

SubPkgA is a standalone package that developed independently (e.g., ImageCore, ImageFiltering), and GiantPkg is an umbrella package (e.g., Images). This helps organize exported symbols in a hierarchical way.

cc: @timholy