fix: swap Span::source_file -> Span::file

rust-lang/rust#54725
rust-lang/rust#139903

Author: Syndelis

macro/src/lib.rs

@@ -376,7 +376,7 @@ impl Paths {
     fn get_call_site_rel() -> PathBuf {
         // Sometimes `proc_macro::Span::call_site()` returns a relative path, sometimes an absolute
         // one. In the latter case, we need to discover the relative part from the project root.
-        let mut call_site_path = proc_macro::Span::call_site().source_file().path();
+        let mut call_site_path: std::path::PathBuf = proc_macro::Span::call_site().file().into();
         call_site_path.set_extension("");
         if call_site_path.is_relative() {
             return call_site_path.to_path_buf();