Explicitly call sys.exit in the modules_mapping generator

When running python with `-S` to disable the `site` module, you need to
explicitly call `sys.exit` instead of the `exit` function which seems to
be a remnant of the REPL

Author: samschlegel

gazelle/modules_mapping/generator.py

@@ -164,4 +164,4 @@ def data_has_purelib_or_platlib(path):
     generator = Generator(
         sys.stderr, args.output_file, args.exclude_patterns, args.include_stub_packages
     )
-    exit(generator.run(args.wheels))
+    sys.exit(generator.run(args.wheels))