Module namespace restructure?

The flat namespace of `astroquery.xx` is getting crowded. Do we want to restructure the namespace to be more hierarchical, either archive-centric or mission-centric?

Start by surveying the existing flat astroquery.xx namespace and documenting how its resources could be grouped. Compare archive-centric and mission-centric organization, then seek a maintainer decision on the target structure; done means an agreed restructuring plan, since the issue does not name files, tests, or a specific implementation scope.

Written by the indexing model from the issue text.