fix(gazelle): Explicitly call sys.exit in the modules_mapping generator (#2662)

When running python with `-S` to disable the `site` module, `exit` isn't
implicitly imported and you need to explicitly call `sys.exit` instead.
Seems to be a remnant of the REPL

gazelle/modules_mapping/generator.py

diff --git a/gazelle/modules_mapping/generator.py b/gazelle/modules_mapping/generator.py
index d5ddca2..ea11f3e 100644
--- a/gazelle/modules_mapping/generator.py
+++ b/gazelle/modules_mapping/generator.py
@@ -164,4 +164,4 @@
     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))