fails to import module from source directory for sources without .py extension

To Reproduce

Given the files:

testdir/integration-tests/common.py

class CommonConfig:
    pass

testdir/integration-tests/test1

#!/usr/bin/env python3
import common
assert __name__ == '__main__'
config = common.CommonConfig()

If I run mypy from testdir/ on integration-tests/test1, it fails to find the import common from the same directory:

$ mypy integration-tests/test1
integration-tests/test1:3: error: Cannot find implementation or library stub for module named "common"  [import-not-found]
integration-tests/test1:3: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
Found 1 error in 1 file (checked 1 source file)

Expected Behavior

I expect mypy to be able to find the common module from the directory containing the source file, and thus to exit without errors:

$ mypy integration-tests/test1

IIUC, it should find the import, since the search path should include: "The directories containing the sources given on the command line"

Other notes

If I rename test1 to test1.py, it works as expected.

If I run mypy from testdir/integration-tests instead of from testdir (such that common.py is in the current working directory), it works as expected.

Environment

$ python3 --version
Python 3.10.12
$ mypy --version
mypy 1.19.1 (compiled: yes)

testdir/integration-tests/common.py と拡張子のない testdir/integration-tests/test1 で問題を再現し、その後 testdir から mypy を実行します。.py 拡張子のないソースに対して mypy が検索パスをどのように構築するかを追跡します。import common が見つかり、mypy がエラーなしで終了し、.py の場合と別の作業ディレクトリの場合も引き続き動作すれば完了です。