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

src/macros.rs

@@ -1091,7 +1091,7 @@ fn next_space(tok: &TokenKind) -> SpaceState {
         | TokenKind::DotDotEq
         | TokenKind::Question => SpaceState::Punctuation,
 
-        TokenKind::ModSep
+        TokenKind::PathSep
         | TokenKind::Pound
         | TokenKind::Dollar
         | TokenKind::OpenDelim(_)