Make the imports of the sql module less complicated

[svaningelgem]

The import structure within the sql module is complicated and not great at the moment.

The core classes outside of the sql module should be fine.

This issue is to check whether these imports outside of top-level are really necessary for those cases like this one?
==> Rationalize & reduce complexity in imports.

Originally posted by @svenkreiss in #152 (comment)