reexport as submodule

I'm not sure if this is doable:

```julia
module GiantPkg

@reexport A using SubPkgA
end
```

that mimics

```julia
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

No file, test, or entry point is named. Start by reviewing the existing re-export behavior and how Julia modules are represented, then determine whether the proposed @reexport A using SubPkgA form can be supported. Done means a decided design and working behavior that organizes SubPkgA's exports under the A submodule.

Written by the indexing model from the issue text.