Call format! through an unambiguous path

[dtolnay]

Repro:

use quote::format_ident;

macro_rules! format {
    () => {};
}

fn main() {
    let _ = format_ident!("foo");
}

Previously:

error: no rules expected the token `"foo"`
 --> src/main.rs:8:13
  |
3 | macro_rules! format {
  | ------------------- when calling this macro
...
8 |     let _ = format_ident!("foo");
  |             ^^^^^^^^^^^^^^^^^^^^ no rules expected this token in macro call
  |
  = note: this error originates in the macro `format_ident` (in Nightly builds, run with -Z macro-backtrace for more info)