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_expand/src/proc_macro_server.rs

@@ -208,7 +208,7 @@ impl FromInternal<(TokenStream, &mut Rustc<'_, '_>)> for Vec<TokenTree<TokenStre
                 Comma => op(","),
                 Semi => op(";"),
                 Colon => op(":"),
-                ModSep => op("::"),
+                PathSep => op("::"),
                 RArrow => op("->"),
                 LArrow => op("<-"),
                 FatArrow => op("=>"),