Fix packaging.database.Distribution.list_distinfo_files (#12785).

This method was supposed to return only the file under the dist-info
directory, but it actually returned all installed files.

The tests didn’t catch this because they were flawed; I updated them.
Thanks to Nadeem Vawda and Jeremy Kloth for testing.

As a bonus, the removal of os.path.relpath use should also fix the
Windows buildbots.

Lib/packaging/database.py

@@ -263,7 +263,9 @@ class Distribution:
         :returns: iterator of paths
         """
         for path, checksum, size in self._get_records(local):
-            yield path
+            # XXX add separator or use real relpath algo
+            if path.startswith(self.path):
+                yield path
 
     def __eq__(self, other):
         return isinstance(other, Distribution) and self.path == other.path