Improve integrity of specs in `emit_sql.py` functions and fields

[bojanserafimov]

A few hasty PRs have broken some of the function and field specs in this module. We should fix that to have a higher confidence in correctness, and to allow implementation of new features without risk.

Some examples:

• _aliases field that doesn't only store aliases anymore
• The concept of used_columns is loosely defined but a lot relies on it