Fix pretty-printing of $crate (take 4)

[petrochenkov]

Pretty-print $crate as crate or crate_name in unstructured tokens like a $crate c in foo!(a $crate c), but only if those tokens are printed as a part of AST pretty-printing, rather than as a standalone token stream.

Fixes #62325
Previous iterations - #56647, #57155, #57915.

[rust-highfive]

r? @pnkfelix

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[petrochenkov]

r? @Mark-Simulacrum

[Mark-Simulacrum]

I think another test here that checks for us doing the right thing with an external crate, i.e., $crate when it resolves to foo vs. crate would be good -- but I'm not sure if we have one already.

With or without that, r=me

[petrochenkov]

@bors r=Mark-Simulacrum

[bors]

📌 Commit 4cb67c0 has been approved by Mark-Simulacrum

[bors]

⌛ Testing commit 4cb67c0 with merge 0324a2b...

[o01eg]

I've got error with serde_derive after this PR:

error[E0433]: failed to resolve: unresolved import
  --> <::syn::token::Token macros>:37:55
   |
1  | / ( abstract ) => { crate :: token :: Abstract } ; ( as ) => {
2  | | crate :: token :: As } ; ( async ) => { crate :: token :: Async } ; ( auto )
3  | | => { crate :: token :: Auto } ; ( become ) => { crate :: token :: Become } ; (
4  | | box ) => { crate :: token :: Box } ; ( break ) => { crate :: token :: Break }
...  |
37 | | => { crate :: token :: Colon2 } ; ( , ) => { crate :: token :: Comma } ; ( / )
   | |                                                       ^^^^^
   | |                                                       |
   | |                                                       unresolved import
   | |                                                       help: a similar path exists: `syn::token`
...  |
54 | | ) => { crate :: token :: Tilde } ; ( _ ) => { crate :: token :: Underscore } ;
55 | | ( $ ) => { crate :: token :: Dollar } ;
   | |_______________________________________- in this expansion of `Token!`
   |
  ::: serde_derive/src/fragment.rs:58:18
   |
58 |                   <Token![,]>::default().to_tokens(out);
   |                    --------- in this macro invocation

error: aborting due to 12 previous errors

For more information about this error, try `rustc --explain E0433`.
error: Could not compile `serde_derive`.

[petrochenkov]

@o01eg
Could you give some minimized / self-contained reproduction?
I'll look what happens.