More setting up to run mypy as per #496.

We need `re2` to be a package, not a module, because it appears that
modules can't have `.pyi` files, so munge the module into a package.

Change-Id: I1a268875743390c32c0fb9cd58f6d83a670ce928
Reviewed-on: https://code-review.googlesource.com/c/re2/+/63270
Reviewed-by: Paul Wankadia <junyer@google.com>
Reviewed-by: Alex Chernyakhovsky <achernya@google.com>

python/setup.py

@@ -3,6 +3,7 @@
 # license that can be found in the LICENSE file.
 
 import os
+import re
 import setuptools
 import setuptools.command.build_ext
 import shutil
@@ -103,6 +104,16 @@ ext_module = setuptools.Extension(
     extra_compile_args=['-fvisibility=hidden'],
 )
 
+# We need `re2` to be a package, not a module, because it appears that
+# modules can't have `.pyi` files, so munge the module into a package.
+os.makedirs('re2')
+with open('re2.py', 'r') as file:
+  contents = file.read()
+contents = re.sub(r'^(?=import _)', 'from . ', contents, flags=re.MULTILINE)
+with open(f're2/__init__.py', 'x') as file:
+  file.write(contents)
+# TODO(junyer): `.pyi` files as per https://github.com/google/re2/issues/496.
+
 setuptools.setup(
     name='google-re2',
     version='1.1.20240601',
@@ -112,7 +123,8 @@ setuptools.setup(
     author='The RE2 Authors',
     author_email='re2-dev@googlegroups.com',
     url='https://github.com/google/re2',
-    py_modules=['re2'],
+    packages=['re2'],
+    ext_package='re2',
     ext_modules=[ext_module],
     classifiers=[
         'Development Status :: 5 - Production/Stable',
@@ -125,3 +137,5 @@ setuptools.setup(
     cmdclass={'build_ext': BuildExt},
     python_requires='~=3.8',
 )
+
+shutil.rmtree('re2')