Fix cache-convert (python#17974)

Tested via:
```
set -x
rm -rf .mypy_cache
time python -m mypy --config-file mypy_self_check.ini -p mypy
python misc/convert-cache.py --to-sqlite .mypy_cache/3.8
time python -m mypy --config-file mypy_self_check.ini -p mypy --sqlite-cache
time python -m mypy --config-file mypy_self_check.ini -p mypy --sqlite-cache
```

mypy/metastore.py

@@ -131,7 +131,7 @@ def list_all(self) -> Iterable[str]:
         for dir, _, files in os.walk(self.cache_dir_prefix):
             dir = os.path.relpath(dir, self.cache_dir_prefix)
             for file in files:
-                yield os.path.join(dir, file)
+                yield os.path.normpath(os.path.join(dir, file))
 
 
 SCHEMA = """