Rename and split some modules in the query system

[Zoxc]

This renames rustc_query_system::query::plumbing to execution and splits rustc_middle::query::plumbing into macros and system modules. This reduces the modules named plumbing in the query system from 3 to 1.

[rustbot]

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[Zoxc]

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

[Zoxc]

cc @nnethercote @Zalathar

[rust-bors]

💔 Test for 99b5429 failed: CI

[Zalathar]

I like the motivation (not having three plumbing modules, and splitting things up a bit), but I'm concerned that these particular names/splits might end up being differently-confusing instead of better.

I'll try to go into more detail later when I have time.

[nnethercote]

r? @Zalathar

[Zoxc]

I think the execution one is ok, as that is the main supporting part for execution queries. I'm not too worried about the other being side grades, given that we probably want some reorganization if we merge rustc_query_system anyway.