Separate the Res::Local case from hir::ExprKind::Path.

[eddyb]

All other things an expression can refer to, have DefIds.
If we separate them, then we can use a proper HirId for hir::ExprKind::Local, and maybe even refactor the current hir::def::Res enum away.

Somewhat related to #53553.

cc @nikomatsakis @michaelwoerister

[ljedrz]

I would like to work on this, but I'd need some guidance; ExprKind has a Path variant, but that one contains QPath that's either Resolved or TypeRelative. Have the names changed, does this relationship with Def::{Local, Upvar} go deeper or am I missing something?

In other words, if anyone could drop me a "change A to B and fix the fallout" hint, I could give it a shot.

[eddyb]

It's impossible to have a TypeRelative path that points to a local variable.
But I doubt this refactor is a good idea now, with all of the other refactors that happened.

cc @Centril @petrochenkov