reorganize query planning and execution

[chewselene]

Moving query planning and execution to its own directory (name is lengthy - open to shorter suggestions!). Reorganize make_query_plan so that public methods live in the Public API section.

Part of the refactor in #1008

[obi1kenobi]

How about just query_planning as the module name? I think one wouldn't be particularly surprised by the ability to execute the planned queries within that module.

[obi1kenobi]

You can run typing_copilot init --overwrite to bypass the mypy and typing copilot CI blocks, otherwise they'll want you to type-hint all the code you moved, which is getting significantly refactored anyway in an immediate-future PR.