Rollup merge of #123462 - fmease:rn-mod-sep-to-path-sep, r=nnethercote

Cleanup: Rename `ModSep` to `PathSep`

`::` is usually referred to as the *path separator* (citation needed).

The existing name `ModSep` for *module separator* is a bit misleading since it in fact separates the segments of arbitrary path segments, not only ones resolving to modules. Let me just give a shout-out to associated items (`T::Assoc`, `<Ty as Trait>::function`) and enum variants (`Option::None`).

Motivation: Reduce friction for new contributors, prevent potential confusion.

cc `@petrochenkov`
r? nnethercote or compiler

compiler/rustc_ast_pretty/src/pprust/state.rs

@@ -893,7 +893,7 @@ pub trait PrintState<'a>: std::ops::Deref<Target = pp::Printer> + std::ops::Dere
             token::Comma => ",".into(),
             token::Semi => ";".into(),
             token::Colon => ":".into(),
-            token::ModSep => "::".into(),
+            token::PathSep => "::".into(),
             token::RArrow => "->".into(),
             token::LArrow => "<-".into(),
             token::FatArrow => "=>".into(),