Add flag to not add top-level modules to PYTHONPATH

stub_template_host.txt added all the top-level modules to the
PYTHONPATH, which isn't correct, and caused absl.logging to
override the built-in logging module.

Removing this also makes it more consistent with python binaries
built with embedded_launcher: true. embedded_launcher: true
binaries don't add the top-level modules.

Fixes: 245583294
Test: m py_dont_add_top_level_dirs_test && out/host/linux-x86/testcases/py_dont_add_top_level_dirs_test/x86_64/py_dont_add_top_level_dirs_test
Change-Id: Id3069565d2b2c4b2bda0ff5301e757a7b4201751

python/tests/top_level_dirs/main.py

@@ -0,0 +1,17 @@
+import unittest
+import sys
+
+print(sys.path, file=sys.stderr)
+
+class TestProtoWithPkgPath(unittest.TestCase):
+
+    def test_cant_import_mymodule_directly(self):
+        with self.assertRaises(ImportError):
+            import mymodule
+
+    def test_can_import_mymodule_by_parent_package(self):
+        import mypkg.mymodule
+
+
+if __name__ == '__main__':
+    unittest.main()