Test that every python module can be imported on its own

[msullivan]

... and fix the ones that can't, which I'm sure there are many.

This will make sure that our import cycles are resilient to being imported in any order.
Something that would have been caught by this caused LSP to break (see #8307)

[aljazerzen]

This will be a hard nut to crack - there is a lot of cross-references between modules.

Maybe we should first start by pulling a module out of the loop, so that it does not refer to any other modules anymore.

For example, edb/schema/ is a data structure that should be able to exist on its own. So we could remove all external references it contains (to edb/pgsql and edb/edgeql). That would require moving DDL code out into a separate module.