Rephrase misleading Multiple directories found assertion in _find_info_directory (#5160)

Author: ooooo-create

setuptools/build_meta.py

@@ -357,7 +357,9 @@ def _find_info_directory(self, metadata_directory: StrPath, suffix: str) -> Path
             candidates = [f for f in dirs if f.endswith(suffix)]
 
             if len(candidates) != 0 or len(dirs) != 1:
-                assert len(candidates) == 1, f"Multiple {suffix} directories found"
+                assert len(candidates) == 1, (
+                    f"Exactly one {suffix} should have been produced, but found {len(candidates)}: {candidates}"
+                )
                 return Path(parent, candidates[0])
 
         msg = f"No {suffix} directory found in {metadata_directory}"