Auto merge of #32432 - eddyb:dock-service, r=nikomatsakis

Auto merge of #32432 - eddyb:dock-service, r=nikomatsakis

Flatten rustc and rustc_trans module hierarchy slightly.

The following moves were made, in the name of sanity/simplicity:
* `rustc::middle::{cfg, infer, traits, ty}` to `rustc::{cfg, infer, traits, ty}`
* `rustc::middle::subst` to `rustc::ty::subst`
* `rustc_trans::trans::*` to `rustc_trans::*`
* `rustc_trans::save` to `rustc_save_analysis` (cc @nrc)

I've rebased a larger WIP branch on top of this and the only conflicts were in imports, but YMMV.