Rollup merge of rust-lang#90376 - oli-obk:🧹, r=spastorino

Various cleanups around opaque types

Best reviewed commit by commit.

This PR has no functional changes.

Mostly it's moving logic from an extension trait in rustc_trait_selection to inherent impls on rustc_infer.

Author: matthiaskrgr

compiler/rustc_borrowck/src/type_check/mod.rs

@@ -36,7 +36,6 @@ use rustc_span::def_id::CRATE_DEF_ID;
 use rustc_span::{Span, DUMMY_SP};
 use rustc_target::abi::VariantIdx;
 use rustc_trait_selection::infer::InferCtxtExt as _;
-use rustc_trait_selection::opaque_types::InferCtxtExt;
 use rustc_trait_selection::traits::error_reporting::InferCtxtExt as _;
 use rustc_trait_selection::traits::query::type_op;
 use rustc_trait_selection::traits::query::type_op::custom::CustomTypeOp;